This is something odd. At first I thought it is just simply not possible to have more then two members in an alliance. Now I was trying to get the Riftborn Steam achievment which requires me to have at least 3 members in my Alliance. For the love of god I can't figure out how to add more members to my alliance and I have been told to post a bug report since everybody else seems to be able to get more members into their alliances. It has been like that with EVERY game I have played so far; I can't add more members to my alliance even though both me and the other player have the required tech and are at peace.

Here is the savegame: Phil Riftborn.zip

This is not a bug  (I Beleve)

The Issue is that you have only 5 major empires which sets alliances to be a maxamium of  2 Empires

From %InstallFolder%\Endless Space 2\Public\Registry.xml (Line 151):

<Alliance>
     <MaxEmpires>Floor($(MajorEmpiresCount) * 0.5)</MaxEmpires>
</Alliance>

The code from the file takes MajorEmpiresCount as 5 to get Floor(5 * 0.5) which becomes Floor(2.5) (Floor generally just means round down) which gives us <MaxEmpires>2</MaxEmpires>

So the game is acting as it should. you can if you want go to the registry.xml file and edit the code (change the 0.5 to 1) 

Hope that helps, The alliance limit is not that well comunicated by the game to the player (I can't remember it saying anthing in-game about it). 

Oh ok thanks alot...now I have to replay Riftborn on a huge map though ;-)