Hello Glenn

Sorry for the late response.

To be able to link Amelia Customers or Amelia Employees with existing WordPress users, those users need to have the "Amelia Customer" or the "Amelia Employee" user role.

Since you're using a Membership plugin, you don't need to add additional roles for your customers, but only have the "Automatically Create Amelia Customer user" enabled in Settings/Roles/Customer. When they book an appointment, while they're logged into your website, Amelia will automatically add the "Amelia Customer" user role to the existing user, and log them into Amelia's Customer Panel. Also, when a member logs into the Customer Panel, he'd also be logged into the Membership plugin.

Please take a look at our video about this: https://youtu.be/nm7f7RDSYow

For employees, though, you need to install a plugin like "User Role Editor" which would allow you to assign multiple user roles to your existing users, so if you want a user to become an employee in Amelia, you need to edit that user in WP and assign the "Amelia Employee" user role to him/her. 

An important thing to note: Do not assign multiple roles to the Administrators, as that will cause issues with capabilities and permissions.