When someone turns a gig down, the gig page shows their chair under Open chairs, and who could cover them, with a box for each open chair.
You can offer the same chair to more than one person. The first yes gets it. Everyone else is told the chair is filled, and their page says so. You get an email saying who took it.
If the player who turned it down changes their mind first, anyone you offered their chair to is told it's filled.
Subs you've used are listed at the bottom of the Roster under Subs from outside the band. Press Add to the roster to keep one. Youth groups add subs to the roster first, with a parent or guardian.