Magic Bands are linked to a specific individual's
MDE account when they are produced. If you have "Bobby" on your room reservation and a Magic Band is delivered, that band belongs to Bobby. Even if Bobby drops off your reservation, the band still tied to Bobby's MDE account, along with any park tickets, FastPasses, dining reservations, etc. which were linked to his account.
To my knowledge, there is no way to take anything which was originally assigned to "Bobby" and move it to another individual.
If you add "Jimmy" to your resort reservation at the last minute, any MagicBands that he already owns should open the hotel room door. But if you're past the MB shipping date, it will likely be too late to order a new custom MB for that trip. Jimmy can still get a gray one upon arrival.