Right now, UpKeep allows access to 1 account per email address. If you and your coworker have already set up 2 separate UpKeep accounts and are now trying to link them together, you'll need to go ahead and delete one of those accounts first and then re-add them to your account.
Here's how you can do that:
1 - Login to the account you'd like to delete
2 - Go to the account settings page
3 - Scroll to the bottom where it says "Delete Account"
4 - Once you have done that, have the other person who previously set up UpKeep, add you to their account. They can do this by going to the "People and Teams" page and then pressing the "+" button to add you to their account.
5 - Viola!