Yes you can, whether it be on the Zone or by IP with friends. You and those that you intend to fly with in these missions will need a program called Multiswap, available here: http://www.multicfs.com ;   **Go to...  Downloads-Utilities-CFS2

Once this program is installed, very easy, you and your friends go online. BEFORE you enter the CFS2 Ready Room either on IP or on the Zone, start the Multiswap program. Then you and your buds select the mission you which to swap to and the allegiance. Select to SWAP and then you can all go into the Ready Room, select acft. and start flying. ALWAYS remember to swap back to the ORIGINAL CFS2 mission before closing down Multiswap.

Now, besides the stock CFS2 missions you and your friends can fly together, there or tons of missions you can all download to fly together. Many are here and many more at other sites. But you and whoever you fly with must have the same mission swapped in Multiplayer or you will see different things.

Hope this answered your question. Good luck and have fun.