When players enter the district, they're 'Off Duty' or 'unavailable' or however you want to think about it. Once they're done with checking mail or sorting their inventory, they hit a key and switch to 'On Duty'. From that point on, they are at the beck and call of their
Organisation, and the matchmaking system decides who goes where and does what. Participation in individual missions is no longer a choice. Your Organisation is sending you to do a job, so get with the programme. After the mission, if you need to get more ammo or respawn your vehicle, just flip back to 'Off Duty' and nobody will bother you until you're ready to go again.