If you are trying to accept an invitation and receive the message “This invitation has already been accepted”, the invitation was likely sent to multiple email addresses.
Whichever email the invitation is accepted under first will become the primary account holder for that profile once a username and password is created. The remaining email(s) would then need to be set up as contact(s) so they can have their own username for the account. If a single contact is listed with multiple emails, it may be that the invitation was auto-accepted under an existing TeamSnap account associated with one of their emails. If you experience this error, try logging in with any other emails you may have associated with TeamSnap or request that the primary account holder add you as a contact and send you a new invitation.