What exactly have you changed for login? Do you create your users in Admin Console or use LDAP integration?
I use Openfire with a local database and create my users in Admin Console. I add full name into the Name field. So when i share the group all users are shown with their full names in the roster. When i join the group chat it uses the same name as a nickname for that chat. Don’t have to do anything for that. Maybe that depends on room settings, but i can’t think of any setting which can affect that. You can post settings of your room here (can be a screenshot).
I forgot one thing. When we are setting up Spark for a user at his desk, after entering username, password, server and logging in, we also go to Spark>Profile and put his Full name into Nickname field. This way when a user is chatting he is seeing his full name in outgoing messages and that also makes Spark use full name in group chats. By XMPP standard a room is looking for the nickname field to use when user joins. As this nickname field is preserved after you delete Spark settings, it is saved somewhere on the server (vCard maybe). So it should be possible to set this field from the server, but there is no field for that.
I have filed a feature request for this (Openfire) OF-1077
Just FYI, i have just tried to add such field myself and… (i’m not a java developer btw) it is not as trivial as it seems. It involves editing 15+ source files, adding a bunch of methods, variables, editing lots of web forms and i still couldn’t make it work