Player stuck in the lobby when I invite him

probably a schoolboy error but I’ve sent someone a link to join one of my groups, he’s clicked it and is now sat in the lobby. The only option I get there is to invite him again.
Why isn’t he joining the group proper like the other two members?

Did you invite the third player the same way you invited the other two or did you do anything different during the process?

He used the link in the group settings and I tried inviting him directly by name

In that case I would suggest uploading a ticket with the group and the player’s names so that they can help you.

Yes, we’ll be better able to assist you with a support ticket. If you can include the name of the group and your player’s username in that ticket, that will be all the information we need to get the ball rolling:

Recently had an issue with this as well; my player was I believe trying to use Firefox? Had no issues when they opened the link in Edge.