How do I form alliances?

To clarify, I can't find any options in the diplomacy screen.

Your relations have to be 'Close' to form an alliance.
Once it reaches 'Close' the option will show up in the trade window if you have the tech researched.

So I have to get them to like me quite a bit first? Thanks!

Yes, and it is not easy.

It is nearly impossible to get close relationships now. Since around version 1.3 of DL it has been getting progressively harder.

I guess I'll never research that skill again.

Out of curiosity, since this is my first game of GC2, how do I get the relationship to "close"? So far I've been giving one computer player about $100 per turn. Will that help at all?

What if I only give him $1/turn? Thanks!

Trade helps, as does being the same alignment.
It is better if you are not too close on the map, because influence spillover can be detrimental. You'll want some military, but not too much - you can't appear either too weak or too strong.

Giving money and techs seems to help, but only to a certain degree.

Thanks.

You can also have a look at each civilization's report to see what factors are currently making them like/dislike you (ranging from ++ to --) so you see what you should and shouldn't change.

Getting close relations is also tougher on higher difficulties: on Normal it happened relatively often even in small galaxies, while on Challenging you already have to try hard.

Avoiding the -- relations categories is a good start. Every time you conquer a civilization or even a minor neutral, it will make it harder for you to ally with the remaining empires. If your influence area covers some of a faction's planets (the --Close Borders penalty), you will have a hard time with them, etc.

The best way to bring the other empires to Close relations is to build up a massive military rating - to the point where your military is at the top of the graph and all the others are huddled at the bottom. Keep that up long enough, and they will start to grovel and move to Close relations.

It has been reported that you partner to be would also need the Alliances technology. If they don't know what an alliance is, there is no point in proposing one.

You can't make them get close by giving them presents. You *can* get them friendly which may help obtain close. The best thing I've found (as stated above) is to build a much larger militar so they grovel with close relations. If they don't have the Alliances tech and they are close, you can give them the tech and form an alliance in the same meeting, important in Dark Avatar where you can only see them once every couple months.

Just to clarify a few things mentioned but maybe not made crystal clear....

Both parties have to have alliance...you can trade that knowledge once you have it...or give it to the party that you would like to ally with.

Trade does help....more trade helps more...make sure your trade agreements show ++ and not just a +

You can check with an ai every week, so long as no transactions occur between you....you are limited in DA to one transaction per couple of months. SO if you chose to trade stuff, do all that you want in one transaction/setting, because it will be several turns before you get another chance.

Check your relations with your potential ally often....it changes significantly from turn to turn....I watched a potential ally that was in a war go from close to neutral in just a few turns, wishing i had offered alliance while he was in the war, rather than waiting to find that that ability went away.

Giving military ships or other ships is also a relationship enhancer.

Thanks!

Will it help at all if I give the AI $1?

Also, what's the monetary unit in this game?

$1 will be barely noticed (if at all) by the AI, especially considering that giving technologies worth a few thousand at that time won't even change their stance right away. I don't know though whether it's better to give them all at once, or spread it across several months.
I think the monetary unit is bc (billion credits)