I've created a test account and looks like the issue might be related to two things:
The invite email takes users to the list where they can accept invites, but once the invite accepted this little message is displayed instead of an error message that stands out.
I had missed the activation email as well. Going through my emails fixes the missing confirmation issue.
This email confirmation message wasn't visible to me after I signed up so I've missed it. If you just make it more prominent that it stands out and I would have known to look for the email in the first place.
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Team invite sign up issues