Right now, UpKeep allows access to 1 account per email address. So if you are looking to move somebody from one account to another you would need to delete them and add them in to the new account. So if you're having trouble adding a user they might already have another UpKeep account that they are a part of.
TIP: If you try to add a user to your team and run into this error message like the one below, but cannot find them in your People & Teams section, that user most likely has created their own account separate from yours.
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.
In order to fix this and add a user that already exists:
Delete their own account first.
This can be done by logging in > Profile (circle button with the silhouette of a head found in the top right) > click on the 3 vertical dots > Delete Account.
Tip: if they can't remember their password, they can reset it here!
Be invited to your team. You can invite them to your team once their account is deleted. Here is a guide.
To recap, to add a user that has already created an account, they need to delete that separate account by logging in to it. Once they've deleted their account, they will need to wait for an email invitation to set up their account.