The effects of grouping on missions?

I was wondering if grouping with another player had any effect on missions.  Here are some example scenarios:

Example 1:  You group with another player, and together you complete a mission of his that you yourself have not even picked up yet.  Will you:

1) ..get the end $information, experience and faction rewards for completing the mission?
2) ..be unable to take that mission from a contact for yourself while solo at a later date, because it has now been flagged as being completed by you?

Example 2:  You and another player have the same mission in your mission journal, albeit at different locations.  You then form a group.  In this case, will you be able to complete the mission for both of you by going to either location?  Or do you have to go to both locations to complete the mission for both members of the team (at which point you're essentially completing the same mission twice).

Example 3:  You and another player group together and one of you picks up a mission from a contact.  Does the mission get inserted into the mission log of both players? 

Thanks a lot for the help.

Do missions scale in difficulty as you add more members to your team (i.e. more enemy NPCs are spawned within the mission)?  Or is adjusting the mission to "hard" and increasing the NPCs level the only way to make a mission more difficult?