There are tons of way to have group chats, and a newcomer (at least to me) is Bindle Chat. Offering an iOS and Android client, Bindle allows you to create private and public chat rooms based around a hashtag.
In my limited testing, it appears to work pretty well, and could be used for various mobile activites. A downside is that there isn’t any sort of desktop client for now.
Try it out, I have two channels up, #eduk8me and #tlah, both should let anyone chat.