Hello I am the owner of a mobile app Club , I cannot send a invitation to a player who is already in a club! If I’m playing someone who is not in a club, and I click on their profile , a blue box will be there that says Invite to club! If they are in a club that box does not open just the name of the club they are currently in. So, you can send a friend request, explain to them they must go to their club home page, click on tools, click on quit club, when that is done and you have them as a friend the blue box will than open up to invite them in!
I hope this helps, By the way anyone wanting to join my Club Toronto Leaf Fans , we’re currently at level 10, a great group of players from all over the world! Not everyone is a Hockey fan, some have only seen ice in their drinks! Lol Sorry had to get my clubs plug in there !