I'm not surprised XSCE and IIAB haven't been able to keep up with how to run XMPP server infrastructure for Sugar desktops. They've not been engaged with Sugar Labs for a while, and I've seen no testing reports.
George Hunt is correct that a special version of ejabberd was used, and that the registration menu item has nothing to do with collaboration, and everything to do with backups.
You might not need an XMPP server, as Sugar has XMPP over link-local support, and this works in class size groups. Both server and link-local XMPP support in Sugar are exposed to the user in the same way; through the collaboration features.
https://help.sugarlabs.org/ has a section on collaborating within Sugar.
To test with an XMPP server, Sugar Labs runs jabber.sugarlabs.org which can be set in My Settings, Network, Server.
To deploy this server on your own infrastructure, use the Prosidy XMPP server software, with the Sugar plugin. jabber.sugarlabs.org is running 0c07a74503031fecd7ebcde7ff1c0cb2e0674001 of https://github.com/bjc/prosody with 223fcec5308435071556a45851ef62ffa587f4d1 of https://github.com:sugarlabs/prosody-sugar and then set your server host name or IP in My Settings, Network, Server.
Also, please consider using our sugar-devel@ mailing list, via https://lists.sugarlabs.org/ because it has way more people reading than this frankly dead discourse forum.