Hey,
If they are already in their own team the easiest way is for them to close their team and then they should be able to join yours/ get the invite. There is a button to close your team in the team or profile page.
Let me know if that worked or not.
If it's not solved, or for anyone else having issues, please send me the list of people & email addresses that want to join your team before Monday (you can private email it) and as long as that comes in before Monday it's fine and we can add them to your team manually next week.
Best,
Nick