Error adding a member in workroom: "Could not send invites to some of the recipients..."

[RF1111]

I am trying to add a member to a workroom, and keep getting the message:

"Could not send invites to some of the recipients: <email_address>"

The email address is valid, and setup as an Oculus account. It is being used for other workrooms, and can enter the workroom as a guest.

Why can't I add it as a member?

Thanks.