For quite some time now the Skyrim community had been begging the developers to add a co-op mode to their favourite game. Well, the developers took notice and will be bringing Skyrim Together, a mod that lets you play online with friends. Skyrim Together will allow for co-op mode that will by connect players to a master server, after which points players can join lobbies within that master server.
The devs promised to make an API after release to further ensure that mods can work in correlation with Skyrim Together although the mod made with this current creation kit is likely to work just as fine.
The mod is expected to come out sometime by the end of the year.