One of the biggest issues with membeship systems and WordPress MultiSite is that a user is setup on a global scale (though they may be limited in access to a specific site).
Setting up users using the WordPress user system is a fine to continue working this way, because as a developer, I have some clients that have multiple sites and can use this to have the same login for multiple users on multiple sites.
The membership plugin is more site specific though ( not necessarily in the scenario of a user having multiple sites though), especially for sites with different owners.
I think it would be great if the membership plugin where modified to work in a way that multiple sites don’t share the same users.
My proposal would be tthe following:
1) For all sites (except the main network site), have a field that is appended to a username before the user is created. This same value is checked as well when a user logs in and appended to the username logged in with.
2) The value in item 1 can be modified by site admins to a value that could be used on all of their other sites, allowing members to be added elsewhere.
3) Maybe a check for number 2 to ensure a unique value among different site admins so that two different site admins don’t use the same value, but a single site admin (by global user login) can re-use the value for item 2.
For my use case, many of my clients use a membership system (Not all use Membership 2) and it would be great to keep their members specific to their system(s)