There are a couple different solutions to this problem. The first is to have that user sign up with another email address. They can use the original invite link and use another email address. This way they'll have two separate logins.
The other solution is to cancel their current account and then sign up again with the invite link. Keep in mind, when they cancel their account, all of their projects and prototypes will be deleted. If they want to keep them, they'll need to export their prototypes prior to canceling. Once they've canceled, their email address is freed-up in the system and they can register with the invite link.