I had previously used the irc gateway, and erc did the trick with no extra configuration at all. But things changed and now I only have the jabber gateway at my disposal.
The problem is that if you want to use a decent emacs client, you'll have to use bitlbee and end up mapping jabber to irc, so you can use erc/circe/rcirc (which are far better at IRC protocol than jabber.el is at xmpp)
To chat privately other peers it's a matter of /j name-of-the-buddy. Joining rooms is not as direct, and first you have to create some 'mapping' rooms in your bitblee configuration, before being able to join them.
Whatever, here's how to integrate slack with bitlbee (and with erc)
On bitlbee buffer:
- chat add 0 firstname.lastname@example.org
- chat list
- /join room-name
- channel 4 del
And that's it. Now you can slack from erc, and you can use all erc machinery (yes erc-robot, I'm looking at you).