Importing Contacts from Groups is a much easier way to move your Contacts into Membership Levels.
Once you've got your Contacts into Groups, you can use Memberships > Bulk Import From Group to populate your Memberships.
Choose your Membership Level you wish to populate:
Importing current Members
When importing your Contacts, you might want to edit the Membership Level to have a zero Membership Fee (temporarily) and also make it "In-Active" (so that people don't join at zero cost).
Making the Membership Fee zero avoids issues with recording payments or sending invoices to people who have already paid and are already members.
If you don't, you'll be presented with this screen when importing:
Once you've imported your Contacts and you're ready to accept online Membership payments, you can make your Membership Level Active and edit the Membership Price to its correct value.