Why am I getting message "The following customers could not be added" when trying to add a user?

[Colleen]

I have a Service Project that I want to add external customers to, just using their email, but it's giving me that error. There are already many people on the team so I know this was working for my predecessor. I go...

Project --> Customers --> Team (group of customers) --> Add customers --> Enter email

Am I doing something wrong or missing any steps? Also, I am certain the email is valid. 

[Grigory Salnikov]

Hello @Colleen !

This would usually mean that these particular users are already present but inactive. 

Could you please check that?

[Jack Brickey]

Or maybe a malformed email?