When does a meal with a few friends turn into a meal with an entire group? They say that two is company and three is a crowd, but I don't think of three as a crowd. Three is a small group, but it is still intimate; four can be very close too, though there is a potential for two conversation threads running in parallel and never touching. But five... Five people is definitely a group. It can still be good, but it takes another level of involvement for me.
I like people, I love my friends, I enjoy spending time with them. But innately, I don't like groups. Or rather, I can and do enjoy groups, but they do not feel natural to me. I get overwhelmed. My focus shifts from the specific people I am interacting with and being intimate with, to the overall dynamic of the group--what kind of conversation we are having (not who we are), who is talking more or less (not what they are saying), how the different members in the group are connected, and who is more closely connected to who... It's hard for me to be fully present in a group (without some activity going on).