can't do missions together, i mean you can be same place with your friends but the mission will be completed separately, like mercenary where need to kill eranin/federation/pirates, you can both accept same mission but it will count kills separately.
together you can pretty much everything where you are in same instance, flying, exploring persistent (no USS) locations, attacking common enemies, pirating npc/pc ships etc.
in current form where we cannot slave our drives and jump/supercruise in group as well as we cannot interdict others so you both need to be at the same persistent location and everything you can do solo you can do together with your friends
yes, drive slaving, coop missions etc., so you can be together always, you can interdict others in group and be as a group when someone interdicts anyone from your group etc.
but this all is coming, you should check design discussion archive if didn't already to know what game should have and how things will work out (or check the links from my signature