Slitherine

That pretty much speaks for itself does anyone know?

It takes a long time. You need to improve your relations to a very high level. Use all the positive aspects of the diplomat you sent to the nation. It costs a lot of money too...

Actually, if you send a diplomat to another faction, their attitude toward you will improve slowly over time. Assuming that there are no situations which lead to an attack by either side (easy opportunity to grab resources, an underguarded city, scenario objective, etc.), you should eventually reach a point where they become an ally. If you share a lot of border, it may be less likely, since there's more chance of the AI deciding to take a city from you.

There are also specific diplomatic missions which raise your relations with the selected faction. Offering mutual asylum (protection from attack) for merchants improves relations over a few turns and also gives you a temporary price break on trade, which allows you recover much of the cost of the mission.

but how do you become officially allied where they attack our enemies?

You can't. Sucks, huh?