If your contact is already portal registered then they won't need to portal register again if you add a new client file for them. You can add any new client files you create to their existing portal.

To add a new client file navigate to Clients > New Client under Main or Secondary Contact click the Person field and search for the person who's portal you want to add this client file too and select them here. Add the rest of the details and click Add Client, this client file will now be available via their portal. 

To check what client files they will have access to on their portal navigate to Clients > People > Registered on Portal, here their name is listed on the left and to the right of their name is listed the client files they'll have access to on their portal.