Internet Relay Chat (IRC) used to be my preferred group messaging app thing, I have IRC running on Vor, using ZNC to bounce IRC to Weechat (in Screen on Vor), Palaver on my iPhone and iPad, and Textual on my MacBook (see irc on the go).
IRC is awesome for talking to technical people, like TermiSoc, HacMAN, and the ilk, however to have a persistant presance in an IRC channel is a pain to set up. There are services like IRCcloud that make it easer, however they don’t make it easy, or are free. IRC also doesn’t make it easy to share media, or make it that easy to get scrollback (the bits you miss when you are away).
A long time ago when I first moved to Manchester, we used to have a WhatsApp group chat for organising meetups, and I complained that it didn’t have a desktop app. WhatsApp was bought by Facebook, and that gave us all two good reasons to drop it like a poison hot potato; 1. Facebook, 2. Lack of Desktop app.
Telegram has automatic scroll back, you can share all kinds of files with ease (including OpenSCAD files), you can also search by username rather than needing a phone number (mine is @Skippy).
Oh and because it is easy to set up (client for most OSes and Devices, and even Chrome) means that its simple just to get our friends to install it, set it up, and invite them into the mayhem.
If you want to give it a go, have a wonder over to Telegram.me for all your devices, and join in (Telegram seem to mostly have working encryption fixed, it may not be completely working, but its better than plain-text, or trusting Facebook, or Skype).