According: http://developers.imsglobal.org/ext_membership.html
This is a simple service by which a Tool Provider may retrieve a full roster for a context (course) from a Tool Consumer. The roster information for the context can only be read (i.e. it cannot be altered or deleted by the Tool Provider).
This service will allow a Moodle course to automatically retrieve and update the list of course’s members. This is interesting for automatic synchronization of user profiles and also for creating groups or groupings inside a course.
This feature will be configurable, there will be settings to specify when the synchronization is automatic or when is by demand, the teacher request the sync process using a link or button.

Plugin and consumer needs:

LTI Provider plugin:
A new couple of options in the LTI Provider settings for a tool:

• Enable automatic synchronization of course members: If checked, a cron task will retrieve periodically the full roster from the consumer tool (course members).
• Sync period: 30 minutes, 1 hour, 6 hours, 24 hours..."

Consumer tool:
*It should support this extension