for Existing user permission: Require admin approval
A: Existing users can’t invite anyone to your product, regardless of your approved domain settings.
=> How to replicate it?
B: Anyone can still request access to your product for themselves or on behalf of someone else. An organization admin needs to approve the request.
=> What's the relationship or difference between & B in case of no approved domains set for my site?
Refer to: https://support.atlassian.com/user-management/docs/control-how-users-get-access-to-products/
