Besides of the ones referenced in the article, there is also ChatSecure [0] for Android and iOS (with a strong focus on security) and Xabber [1] and yaxim [2] for Android (I am the maintainer of yaxim).
Not sure about ChatSecure, but xabber provides MUC (group chat) support out of the box, and for yaxim there is a beta APK you can get here: https://yaxim.org/muc
The beta's limitations are:
* rather broken invitation handling user interface
[0] https://chatsecure.org/
[1] http://www.xabber.org/
[2] https://yaxim.org/