When you import contacts, you're prompted to also select a Group to add the contacts to.
Using "one off" Group Names can make importing into Groups (and then, Bulk Importing into a Membership Level) easier and trouble free.
For example, if you import multiple member types into the single Members group, then you won't be able to differentiate between types when using bulk importing into Membership Levels.
So, consider naming your initial import groups a unique name, e.g. Import Life Members or Imported Members.
Then, the Bulk Import into the Membership Level can still add the Contacts to the appropriate "real" group, i.e. Members.