If 100% of the group logs out, the group disbands.

One person (the last to officially log out) get "ownership" of the group's instance ID. This one is the one that restarts in the same spot you logged out.

The rest were considered "booted from party" after the party dissolves and get their own instance IDs.

This doesn't happen when you play solo because, chances are, the remaining 4 people are keeping your party in-tact while you relog.

In essence, your party is disbanded when all of your characters D/C because nobody is logged in.

Got it?