Google+ Hangouts operates.

You can only have 10 people in a hangout at a time. you might see X is hanging out with 9 people. that’s 10… “X” + 9.

while the hangout is “live” we show only those who have been there. After it’s done, we list the folks who were there at some point… up to 30 (X + 29). Why 30? uh… technical reasons, not logical ones.

you cannot remove yourself from that list. we did this intentionally. given the nature of the net, we cannot prevent bad behavior. But we can do things to help deter. By leaving your signature on the hangout post, folks know you were there. if someone was trolling, you don’t have to ask “who was that guy?” you can find his signature then block or report abuse. only the owner of the post can delete, and they would then delete the whole post. but, anyone who had joined could have reposted. So even then, the original poster doesn’t control it (which fits in line with our “no master of the hangout” principle).

i know that our design isn’t perfect, and there are down sides and up sides to every choice we make. so we may evolve or change things as we learn more about how hangouts are used. but this is how i think about this aspect of hangouts at this time.

Source: http://googleplustip.com/how-google-hangouts-operates.html