This is a situation that I have been in repeatedly and I don't think it's rude and probably won't stop doing it, but I just got to wondering if other people might thing it is rude.
I'm in a few different set groups of friends who have regular get togethers. Book club for example, among others. It's the same set of people every month and we rotate hosting (at our houses or not, depending on the group). When the event is scheduled to be at my house, I figure I have the liberty of inviting an extra guest who is not actually a regular member of the group. Now, these groups are very friendly and open and easy going. And the guest is someone who there is very little chance would be interested in joining as a permanent member so there's no issue of the guest expecting to be invited again when it's not at my house (or even when it is).
Is it rude for me to invite the extra person?
So as a couple of examples:
We have a very casual group who plays poker together now and then. Host serves dinner and we don't play for real money (maybe throw $5 into the pot and winner takes all, that kind of thing). When it was at my house, I invited my sister to come. She wouldn't really want to do it on a regular basis but since it was at my house and she's my sister (and we're pretty close) I thought it would be fun to include her.
I'm hosting book club this week. A friend of mine is going through a bit of a life crisis and has expressed a desire not to sit around her house thinking about it. So I thought, why not invite her to the book club meeting. Again, I'll be serving dinner, there will be a lot of chit chat, we'll discuss the book (which everyone may or may not have read) and it's very casual and easy.
On the other hand, I'm a member of a social group that is kind of, as a tacit agreement, closed. What I mean is, our group is sort of a good size and we mesh well together and it's SOP that we would discuss it amongst ourselves before inviting anyone new to the group. So for that group, I wouldn't invite any outsider on a whim.
Like I said, I probably won't discontinue doing this sometimes as I have not ever had any bad results or anyone saying anything negative, but I was just wondering what others think.