In my job we have a daily chat that everyone must join at 10 AM. I’m using PSI as the xmpp client and that’s easy to do from Psi command line interface with a cronjob:
59 9 * * 1,2,3,4,5 DISPLAY=:0.0 /usr/bin/psi --remote --uri=xmpp:daily@conference.mychatserver.com?join