When migrating an account into a team, the account and all the data in the account becomes managed by the Teams admin (shared or otherwise). Letting an existing account to join a team lets us smoothly support the situation where a user has created a Dropbox account separately and needs to move that account into the team.

For most cases though, users should create a new account for the team. The Dropbox for Teams sign up process guides users towards creating a new account when joining a team for this reason.