This is actually two etiquette situations in one, but I'm going to present both here because one doesn't make much sense without the background of the other. I apologize for the double-length post, but think of it as two for the price of one

background: DH and I have fallen in with a group of friends, most of whom are married with kids about the same age as Babybartfast. We all call ourselves just "The Group" and often have social events open to everyone - the coordinator/host/instigator just sends out an email to the Group and whoever wants to come, does. Obviously we're closer to some couples than others, and there are a few couples we probably wouldn't hang out with at all if they weren't "friends of friends" and on the same mailing list, but all in all it's great.
There are suddenly five other SAHMs in the group (I used to be one of two). This spring, I sent an email to the other five moms asking if they'd be interested in holding a weekly playdate - only two of them have toddlers (Babybartfast is two) who would be old enough to actually "play," but it would give us all a chance to hang out and chat each week. All five said they'd be interested. At the same time, I met another mom "Andrea" - she has both a son Babybartfast's age and an infant daughter, she seemed like she'd be nerdy enough to fit in with the rest of us

After meeting her a few times, I invited her to come to the playdate sometime. (Specifically, I wanted to see if she and her husband would be interested in joining the Group - they're relatively new here and both neat people!)
Our playdates went pretty well for the summer - one of the other Group moms (call her "Kate") almost never came because she had her older boys all day and they wouldn't be interested in hanging around with toddlers, but she did show up once or twice, without her one-year-old, and stay for half an hour or so. As additional background, Kate is kinda related to Bianca, another one of the playdate moms (Kate's and Bianca's husbands are cousins and their families hang out quite a bit).
So
etiquette issue #1: we usually rotate meeting at various people's houses, and each week we'd just say "Who wants to host next week?" and go from there. Usually the host would send out a reminder email to get a headcount the day before, but if that person didn't do it, I would. In early September, I figured it would just be easier to send everyone an email with the next few weeks' dates, and people could sign up for something - this would give Kate a chance to pick a week if she wanted to host. She chose a week I would be out of town, but hey, they don't need me to have fun!

She never sent out a reminder email, and I didn't either since I was out of the state at the time and didn't think about it. When I did check (still on vacation), I found a very PA email from Kate:
As you "might" know. I hosted the playdate this week. [Bianca] attended with a dear friend of mine that just had a baby I was hoping to introduce to everyone. Noone else came and i did not get any emails that anyone had not been able to attend... I was sorely upset about this. I know I am not able to attend alot of the playdates, but that is because I also work. I took great lengths to be home in time to see you all and spend some time together. I signed up for this date months ago when the scheduale went out. Because of this, i am requesting to be removed from everyones list of invites and I will obviously not be hosting any more of these. I am sure you understand why.
Now, if she had actually been coming each week, she would have known that out of the six other playdate moms:
- two had pulled out of the group due to other commitments when fall started
- one pulled out because her son started a special school
- I was out of town
- Andrea had never actually been to Kate's house, didn't know where it was, and Kate never sent out a reminder email or an address!
Even though she hadn't heard this in person, all this information had been communicated by email at some point prior. I sent an apology email and basically said "since three moms have temporarily had to drop out, I think the playgroup has pretty much gone on hiatus for a while." Never heard back, and for all I know, Kate is still mad at me

Leading to
etiquette issue #2: when I was chatting with some of the other moms about when we might be able to re-start the playgroup (and what to do about the Kate situation), they expressed discomfort at having Andrea as part of the group. As much as they all like interacting with her, she just . . . doesn't tend to her kids while in playgroup. In particular, she lets her infant daughter scream and scream, and lets her toddler son run around with a sopping wet diaper, and doesn't do anything about it. Even with hints from other moms like "Ooh, someone's stinky! [Checking own child] Is that you? Nope, Andrea, I think that's [Andrea's son's name]!" Andrea just says "Ooh, probably!" and then doesn't do anything about it. One of the moms even offered to change Andrea's son for her, since Andrea was holding her daughter at the time, but Andrea declined the offer of "help" and her son still stank.
Now honestly, I never really "noticed" this before - I say "noticed" in quotes because once the other moms pointed it out, I realized they were right. And as cute and well-behaved as her kids are and as wonderful a job she may be doing in parenting in other ways, it's really annoying to have a kid "crying it out" while you're trying to chat with your friends. DH and I hosted a Group Halloween party today - Andrea and her family came, and it was the same thing all over again. For various reasons, we only had two other couples (out of a dozen or so in the Group) able to come - it was fun, but not the "get to know everyone in the Group!" I had hoped for.
So the issue here: I can't make the other moms like Andrea. She and her DH aren't on the Group email list yet, although if I forwarded her email on to the guy who maintains it, he'd put them on there. In almost all other respects I think she and her DH would fit in great, and once the kids are out of diapers I expect (based on how well-behaved her toddler usually is) there wouldn't be any problem with her kids fitting in with everyone else's. However, if I keep involving Andrea and her DH in Group events, they'll eventually really need to be on the email list, and once that happens there's no graceful way of saying "Sorry, you irritate everyone else, so you're only invited to events at my house and not anything anyone else is hosting." There's also the issue that I've only known Andrea since this spring - we really hit it off well, but unfortunately some re
lationships sour upon closer acquaintance. Because of the nature of the Group, that would be really awkward. (I will say, as a sidenote, that the few times we've interacted with them while Andrea's DH is present, the stinky diapers have not been a problem - having two parents for two kids helps, and he's good about dealing with issues as they come up. They still let their daughter squall, though.)
So, eHellions, what on earth do I do? I'm probably not going to see Kate all that much anyway - she's one of the ones I probably wouldn't hang out with if we weren't both in the Group - but I don't want her to hate me. At the same time, if she's going to carry a grudge, she's welcome to it. And I don't know a good way to deal with the Andrea situation, particularly when she asks (like she did tonight) if we're going to be starting up our playgroup again

Any advice would be much appreciated!