Hi! I'm trying to add users to a project, this users already have their own account of zoho project, how can I link them to my project??

This users have the free version of Zoho Projects, I have the paid version. 

I already work with other users that also have their own account and i could add them to other proect, but in particular whith this other users I couldn´t.

PLEASE HELP! Thank You in advance!