If you see the following error "This e-mail is already associated with another account. Remove this player and add again with this new e-mail address.", simply follow these steps:
Remove the player from your roster by clicking on their name, and selecting "Remove". This will not remove their BenchApp account, only their connection to your team.
Re-create the player, inputting the same e-mail address that is already associated to their previous BenchApp account. This will now link them properly to your team, using their existing account.