Welcome to our community!
I hope I understand your question correctly.
As long as the course Enrollment End Date has not passed, students who have registered and activated user accounts can enroll themselves. (also the course must not be ‘By Invitation’ in Advanced Settings, but this is the default setting so don’t worry about it).
You can manually activate a user’s account via the Admin panel:
(1) Navigate to /admin (e.g. www.your-domain.org/admin)
(you must be signed into the LMS first to access this admin page.)
(2) Click Users and search for the user you want to activate then click that user.
(3) On the next page, under Permissions tick the Active box.
Common issues with user account activation: If the user has not activate their account themselves, this might be because the activation email/link went to their spam folder.