If you didn't get any join offers, check after the game to see if they exist at all. Go through the codes with archons and bubbles. If there are no join offers, it's a bug, replay the game.

If the suspicion arose during the game, have the 3rd player check the save.

I remind you that before starting the game, you need to disable the "HB rules" checkbox (or better yet, disable HB rules altogether).

If the game is on a mirror (i.e., without HB rules, it won't work), then update the HD mod; Baratortch said he fixed the bug. And just in case, don't use the automatic restart.