Go to Contacts on your left sidebar.
Please choose the user for whom you would like to create an account.
On the bottom left of the contact, click the green button saying Create User Login.
You will notice the newly created user displayed on the right-hand side of your tab.
Select the membership site you made on the first option.
Type in the username and password of your choice then hit submit.
If successful, you will see the user information on the bottom left in place of the Create User Login button from earlier. You can now use this to log in to your membership site.