You would need to have permission on your tenant to create users, this is an Azure AD privilege so you would need to have a global administrator or user administrator role.
Then to assign access in the Azure portal you should be adding role assignments - https://learn.microsoft.com/en-us/azure/role-based-access-control/role-assignments-portal?tabs=current
Classic administrators is a legacy method for older deployments so probably not relevant unless you are still using "classic" Azure resources.