Each player has a Primary instrument, which puts them in a section, and a Usual chair, which is just a name you type.
The first player listed in a section is its lead chair. Use the ▲ and ▼ arrows on the Roster to change the order. The gig page warns you if a lead chair hasn't said yes.
Can also play lists the other instruments someone covers. When a chair opens up, GLAR suggests players who play that part as their main instrument or as a "can also play".
Crew are kept on the roster for your records. They are not asked in date polls and don't get gig offers.