I’m working in a project required new portal users must be approved by system admin, the user filling the User Signup form then Admin assign it to a group and activate it.
Any advice 
Look up at the Workflow and User Permission. Maybe it’ll help 
Assign a role like guest or other role which don’t have desk access in portal setting for default signup users role, after signup admin will see the user and assign a role to it.
assigning a role could be assume as activating.