According to Rockstar North president Leslie Benzies threre's over 500 missions. Some can be completed solo, but others are complicated and require a team.
In missions involving bigger teams a lobby window opens up. From this the roles such as sniper, look-out and transporter can be assigned. The crew leader can tweak settings such as time of day, weather, difficulty and how many lives they have to complete the mission.
You can invite friends to your home and drink, call strippers, watch Rockstar's custom created TV shows (or your friends in high-speed chases).
Also latest GTA V 'o clock:
Take your time with the logo JJJ. Online launches in three weeks anyway so no rush.