How do I add more users to my invoice account?

How do I add more users to my invoice account?

Hi I see that we can now have 3 users for a standard invoice account but how do we add extra users? I have managed to invite a staff member but cant add them to the drop down 'email from' .

To elaborate when sending a invoice or estimate I want staff to be able to choose themselves as the sender of the email from the 'from' email address.

Cheers

Phil