As I see it you only have to let the other player(s) know that you have an alliance, all details (how long it will last, against whom, etc.) can be discussed by PM.
i was slightly confused about this too but in anoter post i was told that this was already discussed on cc and the public saw it not fair to use pms although your alliance has been stated and that all team communications had to be through game chat. i personally disagree with this but this is what i was told and have no proof to support it. (i.e. the link to the actual thread where this was discussed)
petebob wrote:You should drop out of school immediately--it's impinging on your game!