Pending means that the person you've invited hasn't yet activated the account. Via the "Re-Invite" button you can send the invitation again.
You can re-invite user again in 24 hours.
It might be also helpful to inform the person you are adding about the invitation separately, because automated emails sometimes end up as spam.
If for some reason, your player has not received invitation link, you can copy it and send it, for example, via email:
Pending status will disappear once user activates account by creating user name and password.