To sell memberships, you need to create a ticket under Basics (or choose an existing ticket) in your event dashboard.
Then go to ticket settings (Ticket settings for new ticket display after you create and save it for the first time).
Under selected ticket's settings, Enable Membership and enter number of days for which this membership will be valid.
Click on save.
The Membership ticket will now be displayed in the list of tickets on your event website.
Users can now purchase memberships and once the payment is confirmed, their membership details shall display under My Membership accessible from user dashboard on the event website.