I think a secluded building is required. In there, there will be a few NPCs that are hired mercenaries, with one of them being the leader. Once you've talked to the leader, you should be able to interact with the contracts placed on a table behind/near him. From there, you can select which heists/assassinations you want to perform and get warped to a waiting room. Once the required amount of members is archived, then they get warp to the mission area.