If everyone has the exact same mod(s) properly installed and enabled then I don't see any reason why not. Maybe you guys should all log onto Ironclad Online (ICO) first to test it and see if you can join each others' games because I'm certain that you can play mods over ICO. Also see if you can join other people's non-modded regular games.
It's possible that some of you may be suffering from the dreaded Mesh Error--"Your mesh files differ from the host". If so it's easy to fix. (If you log onto ICO and try to join other people's games and don't have any mods enabled and you get a window with that error, it's the Mesh problem.) Just go to this thread and read the last post in the thread:
http://TinyURL.com/Sins10
Do note that it's possible to play Sins with strangers over ICO, so it isn't necessary for everyone to wait until all members of the group are available--you can play online right now by creating an account and user name ("Add Player") on ICO.
Oh, one more thing. Everyone needs to have exact same version of the game in terms of patches. You can patch it and check by using Impulse. (If you can log onto ICO, you are up-to-date.) Also, you can only play together if everyone is playing the same version/expansion (so you can't load the Diplomacy expansion and play against someone who only has the original Sins.)
