I had this occur when a user clicked on 'Accept Invite' in the initial invitation, but was unable to login (either due to their own inability or a connection issue -- they were unable to describe exactly what they did or what happened).
Their user disappeared from our team's 'users' list and i could not re-add them, because they already existed (though they were completely invisible).
I asked them to send me the invite email, and I clicked on the 'Accept' link in it myself.
The next page acknowledged that the invite link had expired, and had an 'OK' button. I clicked that, then I returned to the 'Users' list, refreshed, and her user account appeared.
Is any of the above a sane or reasonable excuse for the user disappearing? No.
But perhaps it reset some cached data or similar, or finalized a workflow that was broken when the user couldn't log in after accepting.
Worth a try to resolve:
- Have the user try to click on the invite 'Accept' link again.
- Or open that link in an different browser.
- Or, get the link and visit it yourself.