You can quickly send invitations to users by entering their email and selecting the role you want to give them from within the aircraft settings page:
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You can easily view all of your outstanding invitations on the same page, just below where you send an invitation. If the user has not accepted it, consider asking them to check their spam or junk email.
You can easily view all of your accepted invitations on the same page, just below where you see pending invitations if they exist, or below where you would send an invitation if there are none pending.