At any time, you can hit the J button to view your Journal and see any missions you may have. The Journal can hold up to twenty missions at once, including sub-missions. It tracks and sorts them according to the location where it was picked up. You may only track or have active 1 mission from that list of twenty.
In the Journal, there is a button (which looks like a crosshair) that allows you to track your progress for that mission, a second button that lets you share certain missions with your team members, and a third button that aborts the mission.
Sharing missions is generally a good idea, because the extra help will make completing the mission easier and faster. It also encourages others to share their missions with you, which means you won’t have to spend as much time searching for them. Certain missions cannot be shared, such as those that require the use of a specific item given by an NPC, or those that are part of a mission chain.