This is really easy, but you need to be lucky who you get online with.
If like me you are just on your own playing online - just go to the social tab on the menu and select the middle tab on the bottom row - Co-op online.
You will get added. The good thing is that you can keep doing this over and over again - the bad thing is that sometimes people in the group aren't doing events, if they aren't, just re-search and you get another one.