Question on Missions

Quick question. Lets say I'm up to the 5th mission in the game and someone wants me to help with the 2nd mission. Am I able to help the person with a mission I've already done?

Also one more thing if I'm on mission 5 and someone groups with me doing mission 10 what happens then? I'm still in the pre-game part but going to see tydis this weekend and was wondering how this works.

Thanks
magichacker

you can go back and replay old missions as long as you're not too high of a level. If someone is on mission 10 and you're on mission 5 however you can't go play with them. They'd have to come help you work up to 10.