Quick reply: I’ve been using XMPP/Jabber and got quite a few people to join, using the Conversations app.
Advantage: it’s decentralised, so if you don’t like how a server or app is behaving, you can switch to a new one and keep all your contacts. You could even spin up your own server, if you’re that enthusiastic.
Disadvantage: all the options can be confusing, so you’d better choose one set (server and client) to refer all your friends to (and maybe set up their accounts for them).
There’s also a spinoff of Conversations called Quicksy which lets users sign up using their phone number, which might make it easier to get some people onboard.
(I have to go now but can come back when I have more time, if you want details)