Its hard to find the mods, maps, guides, etc for things like solo play, AI improvements, team play, servers supporting those, etc. It gives the opinion that no such things exist. A sticky for such things in the Dominions 3 forum, not the MP forum, would by nice (IN MY HUMBLE OPINION before the sparks fly)
But back to your question:
Some things that come to mind are..
Some of the "weak" nations are particularly good for ally play. Ones that are often tossed out of consideration for head-to-head play. Certain magical paths, god builds, built-in abilities such as seduction and stealth and flight, can be very helpful to an ally who can provide defense for the group.
Some tricks have been laid out in some threads that are specific to ally play. What strategies for dominions scales, and global enchantments, can aid allies or should strongly be avoided because they damage allies. How to give specialists such as mages or fortune tellers or healers to your allies. How to setup a province with a castle to be a "turnstyle" province to allow your allies army to move through your territory without leaving it totally undefended.
Also online "whiteboards" can be fun. Where you can upload the map image then all of you can draw on it to lay out strategies and plans like the big planning boards in war rooms in the movies.
