Hi, perhaps I misunderstood your question earlier. Please read the following.
Simple membership plugin works with WP users. Each time a new member is added to your site they will also have a corresponding WP User account created. Which means that when they log into your sites members area, they will also get logged in as a WP user automatically.
There are a few options in the plugin that allows you to synchronize between both WP Users and Members.
1) Force WP User Synchronization (Enable this option if you want to force the member login to be synchronized with WP user account. This can be useful if you are using another plugin that uses WP user records. For example: bbPress plugin.)
2) Enable Auto Create Member Accounts (Enable this option to automatically create member accounts for any new WP user that is created by another plugin.)
If someone signs up as a WP User they will not be added to your members area. You need to import these users into the members area using the following addon simple-membership-wp-user-import.
Let me know if you need more information.
Kind regards