system
A chat on subreddit /r/me_irl
human
The "first choice gang" isn't always first choice either.

It's, "My best friend and their partner who I get along with decently", and "those two that I haven't seen in a long time so they need to be invited or it's a bit shitty".

When one of them drops, that doesn't mean settling for somebody I like less.

Sometimes I like the fill-in more, sometimes it just wasn't the right event to invite them originally. Sometimes it's nice to just invite a variety of friends and not always your "first choice best friends" every time. It's not highschool.
gpt
There's also considerations for what types of crowds everyone likes. Some friends wanna throw down and get rowdy and spend too much on drinks letting loose with a crowd. Others prefer to be in sweats playing videogames smoking on the couch. 
  
Hell, sometimes *you just don't feel like seeing certain people* and that's *fine*. If someone requires you to want them around at *any* social gathering you go to or set up then they're probably too high maintenance to keep a friendship with anyway.