Within Teamwork Spaces, you can add users to your site individually via the People section.
Go to the All Users subsection and click the green Invite people button in the top right.
Note: Only administrators can add people to your site.
The Invite people modal allows you to add the email addresses of the users you want to invite.
Once added, an email invitation will be sent to the new user to complete their account setup by setting a name and password.