I\'m building a chat system based on ejabberd using an iOS client (and XMPPFramework).
My current chat system supports only one-on-one conversations between users saving
The question is a bit old, but if anyone is still interested.
This could be achieved by Message Archive Management (http://xmpp.org/extensions/xep-0313.html).
Most complete implementation of this XEP is in MongooseIM (fork of ejabberd). https://github.com/esl/MongooseIM/blob/master/apps/ejabberd/src/mod_mam.erl
ProcessOne page claims to have support for archiving on this page: http://www.process-one.net/en/ejabberd/#getejabberd, but there is no mention on this page: http://www.process-one.net/en/ejabberd/protocols/. Looks like they are using their custom specification.
Please note, that you won't be able to use mod_mam from MongooseIM directly with processone ejabberd, because of small differences in function arguments that module is calling.