I'm trying to get one off the Steam Achievments and as Riftborn I'm supposed to have an Alliance with 3 other Races. How the hell do I invite someone else to my alliance?

You need a peace with a race  then succed to invite this race in alliance. All you need is in the diplomacy screen/dialogue.

All other alliance will join automatically your alliance (you can't make 2 different alliance afaik, it's one big, and if a race in your alliance create an alliance on his own, it's still your alliance that is used).

I can't check rigth now but can provide you a link to a savegame where I create a 3 races alliance (in fact, it's with 2 different faction of the same race, so maybe count as 1 race, can't be sure)

Sure here it is from both the Unfallen and the Sophons. This is like my fifth game and I NEVER had the option to add someone.

Ok, the only reason I saw is that you don't seach the tech. If you have the tech, then I guess it need a report in the bug section.

I just checked to see if there is some game option that limits the amount of alliance members. There isn't. I'm positive that both me and my partners have the tech. Also I am from time to time getting a message from other empire that they would like an alliance, just not option to actually invite them or accept the proposal. Freaking weird. Posting a bug report...

Hi,

How many players in your game? There is a limit of max 50% of all players in an alliance, so if you're playing a 4-player map, this is probably why you cannot invite him.

Cheers,