[Edi]

#allies <nbr1> <nbr2>

for each pair that you wish to have allied. Going to be a LONG list of #allies commands in the map file if you want ALL AIs to be friendly among each other.

So just start with
#allies 1 2
...
#allies 1 <last nation number in era>
#allies 2 3
...
#allies 2 <last nation number in era>
#allies 3 4
etc.

Once you have that, you can copy-paste it to all map files. Just make sure you don't ally nations 23-25 with anyone or things could get VERY interesting (23 is independents and 24 and 25 are special monsters).