How to register customers on Professional plan

Hi there!

I am having some trouble registering customers for the customer portal, I have the customer portal addon and I have a Professional plan, however it seems that I can only register about 8 users and according to the plan, I have 200 customers per user. I have read and this is what I found about the users and permissions.

" Users and Permissions

Navigate to Users and Permissions and click on “Add Users” tab in the respective permission set to add users to that particular permission set. Alternatively, you can also click on "Add Users" tab on the top right to add users."

However I cannot see this 'Add Users' tab anywhere, and when I try to 'Activate' a user It says me that I have reached the limit and need to buy more.

Also in the Users and Permissions section appears this message:

"Warning! All your profiles are inactive. Kindly Upgrade to Enterprise Edition to use this feature."

So, I need to buy The enterpirse Edition to use the 200 customers per user?(The enterprise gives 500, though) Or In which I got lost and I cannot add any customers.

Thanks for your time!!