You can add a license note to declare the terms of service of your portal and make the users accept it upon login.
In order to do this, as an Administrator goes to the 'System Settings'-> 'Users' section (1) and type the License note in the text area (2).
You can even force the users to accept the license note every time they log in to the system, by checking the relevant option (3).
After enabling the License note option, the users will see the following screen when they log in to the portal:
If they click Decline (4) they will be automatically logged out of the portal. If they Accept the note (5) they will be redirected to their homepage.