In the past, you must be staying at a DVC resort. They scan your magic band to see where you are staying but it isn't exclusive to BLT just any DVC resort.
The rules changed last year. You must be staying on a reservation, either points or cash, at a WDW resort (not including Swan/Dolphin). You and any guests staying with you on your reservation can go up to the lounge. You are expected to use Disney transportation (or a cab) to get there and cannot park at the CR/BLT if you are not a guest there. There are a few blackout dates around July 4th and NYE when only BLT guests who are DVC members staying on DVC points are permitted up. Just being an owner without staying on the property won't do it.
It used to be ONLY people staying at BLT could use the TOWL but they opened it up to all DVC members staying on property. Technically you would have to be staying with the member but in reality, they usually only scan the magic band of one person to confirm a reservation and ask how many people are going up. I've never in all my time had more than one party member get their band scanned in the lobby.
We stayed at SOG two weeks ago and went to TOW. That may have been because it was not crowed at all. Probably depends on the cast member if they scan your band or not. All I remember doing was showing her our member card.
But we did have magic bands on, so maybe she assumed.
I've gone up often with friends who were staying in a different DVC studio than we were in, but were staying on our membership. There never has been a problem.