Within Teamwork, you can upgrade an existing contact to a collaborator so they can log in and be assigned tasks.
To view the People area, click the More option in the main navigation menu on the left of your site and select People.
In the People area, select the People tab from the horizontal menu to view and manage your users.
From the list of people on your site, select the relevant contact to view their profile.
In the top right of the user's profile, click the options button (three dots) and select Upgrade to Collaborator from the dropdown menu.
You will be asked to confirm if you want to upgrade the contact to allow them to log in.
Once you have confirmed, you will be prompted to send an invite email to the new user. You can do this later if you wish by clicking No, Thanks.
Whether you choose to send an email or not, the user will be upgraded to a collaborator.
For more information, see: Downgrading a User to a Contact