Yes. ThinkUp supports multiple ThinkUp users as well as multiple service users (i.e., Twitter, Facebook, or Google+ users). Any ThinkUp user can have any number of service users.
Every ThinkUp installation has at least one user, who is an admin. This person is generally the one who installs ThinkUp and sets up plugins for use.
The ThinkUp admin can open registration to the public so that any new users can create accounts. The ThinkUp admin can also generate invitations to invite specific people into the ThinkUp installation, using a secret, one-time use URL.
Each ThinkUp user can add any number of service users, and may even add multiple users on the same service. For example, a ThinkUp user can add 3 Twitter accounts, 2 Facebook users, and 1 Google+ account into ThinkUp.
We recommend adding a maximum of 6 service user accounts to a single ThinkUp installation.