Just go on to monster galaxy through here, press add neighbours tab and you will be able to add the friends on gaia. Of course if none of them are accepting the request thats a different matter.
Go to the Add Me Requests forum, you can see it at the top of this forum. you should find people there that want neighbours(obviously lol).
There's a bug affecting some people that makes it impossible to do anything neighbor-related, so you might be getting hit by that if the add neighbors tab isn't showing friends you already have that are playing the game.