As an Admin user you can manage users by going to Settings > Users.
From the Users page you can do the following:
Add new users
Delete existing users
Change user roles and rights
If you cannot add a user these might be the causes and solutions:
You might not be an Admin user. Solution: check if you are signed in with the right account, if yes, reach out t your team's admin user and ask if they can make you Admin.
You might not have enough user seats anymore. Solution: delete users or upgrade your plan.