You can add as many team members as is allowed in your pricing plan. To add a new member:
Click on the arrow near the organization name, to open the drop-down list.
Click on “Member & Roles.”
At the member overview page Click "Invite" button.
Enter member's email.
Select a role.
Let's overview permissions here one more time. You have three options :
appoint a new member as an Owner - then he/she will have all accesses;
choose an already created Role (permission set);;
create custom organization permissions for the user.
Custom permission set
You can create the custom permission set for the user and (if needed) add the custom projects' permission for separate projects.
Restricted permission mean you can choose which part of the category user can't access at all, which can view and manage.
Full permission make the category manageable for the user.
No permission close all access for the user to a specific category.