Gosh, I wonder why noone thought to invite Little Miss Sunshine along? She sounds like exactly the type of person who would brighten up a Christmas tree hunting expedition.
I've been left out of functions where it would've been nice to receive an invitation. And yes, my feelings were hurt. I didnt' saything and just let it go but for once, it would've been nice for someone to have said "I'm sorry we didn't think about it but of course you'll be invited next time."
I do agree that Christine should've emailed the hosts directly as the OP was invited along and therefore as an invited guest, couldn't invite Christine either.
Actually, I don't agree with either. Christine should not be emailing the hosts of the event to chastize them for not inviting them or to ask for an invitation either, because doing that is rude.
And for the first - I don't think that hosts are required, by any rules of etiquette, to invite someone to an event that they are hosting because of potential hurt feelings. There are lots of perfectly acceptable reasons for not inviting someone that are completely innocuous - you want to socialize with a specific person, say, or are hosting an event of a manageable size, and don't want to invite everyone of a similar friendship level or relationship
And complaining often backfires. If I have dinner with friends X and Y to chat and catch up, and friend Z complains that's she's hurt because she really wanted to be invited too, then my reaction is usually not to say "Oh, I'm so sorry, I'll invite you next time!", it's to decide that I'm going to be careful not to mention the next time I go out with friends X and Y. Constructive ways of dealing with it would be inviting me, X and Y to a event together, or saying something like "Yeah, we should get together soon" and issuing an invitation.
As an aside, I find these sort of conflicts often occur when people are in the process of transiting out of a high-school/college social style. At younger ages, social events tend to be more informal, bring the whole gang, the more the merrier types of things, where being the only person in the crowed not told about the party or pub night is very noticeable. As you get older, the style tends to change to more focused, invitation specific events, where the dynamic and invitation style are very different, and not being invited is a totally normal thing.
If not being invited is a chronic thing, then it can go several ways. You can simply go with the flow. You can issue more invitations yourself, and see if they are accepted. You can issue less invitations, and back off on the friendship. Or you can evaluate the friendship to see if it's unbalanced - if you care more about it than they do.