There are several reasons why they might have trouble joining your rally. Here are some reasons below:
-The rally is closed and requires an invitation - send them an invitation if this is the case.
-It is possible to join the Facebook group and not be joined in-game. They might just be stuck in one but not the other. We recommend removing them from the Facebook group, then trying to have them rejoin/be reinvited to the rally a second time. It usually works!
-They are still a member of another rally - tell them to leave the rally they are in before trying to join another.
-They just left another rally (they need to wait 24 hours)
-They left a rally but are still a member of the Facebook group of their old rally -tell them to remove themselves from all Facebook rally groups.
-They used to be in this rally - Facebook doesn't allow people to return to rallies they have left sometimes, it's a weird rule. Same if they were booted out. There is no fix for this, so they may have to join another rally.
-This rally is full. Make a spot available for them.
Check to make sure they are not a member of any of these groups.