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what function in the plugin I can use to assign (additional) new membership levels to existing members ?
Yes, I think a way to check the existing user (match email address) then if the user exist, it will update or add a new membership level should be great.
I am using different membership levels to control access to different zone of my website. e.g. membership level A can access posts in category A, and membership level B can access posts in category B. A user can have multiple memberships.