In the staging zone, ACE self-interact → Staging Zone → Group Menu (the same menu has Heal, Arsenal and TP).

The screen lists the groups and their roles, OPEN or TAKEN BY whom, with each role's default loadout. Double-click a role to take it, or select it and press TAKE ROLE. A greyed role is filled; a role that asks for a rank or a skill you do not hold says so - an admin can grant access.

After a respawn you are put back in your old slot, or shown the menu if you had none.

[PICTURE: the TAC//ROLES screen]