Do you see the users in the group in the Buddies list? For me, they appear in the "Offline" group (since that is enabled in Messages prefs), and I see a "Workgroup" group, which I believe is from the group that Server was told to make everyone buddies in the group.
Are you experiencing this issue with iChat on 10.7 or earlier, or Messages in 10.8, or a third party client?
That is the correct place. Have you tried restarting the Messages service on the server? Have you tried removing the Messages account on your client machine and readding it? In the screenshot you gave, is "Amis" an OD group? Just to be sure, the groups and users are stored in the 10.6 OD as network users?
I wonder if it is an issue with how/where this buddy data is stored. I think it is stored on the 10.8 Server in /Library/Server/ like the rest of the server data, but I am not familiar with how the autopopulating works with multiple servers.
No, "Amis" was a personal group.
After posting my previous reply, I played around again, unchecking then checking the option, and wow, a few groups appeared! I'm moving forward!
However, the groups are empty, e.g. no friends, even if my OD groups have many users. Shoud I expect to see my collegues there?
Since this is a new server, these people have not loggued on once before. Do you know if they have to log on at least once in the iChat server before they can appear? (Sorry if I seem lazy, but the environment is not at my workplace and logging in a user is a bit complex.)