How should I modify Risk to play with 7 persons?
We used to play Risk 2210 a lot, often with more than the recommended 5 players, and used the rule that if you are wiped out, when your turn comes around, you can start a rebellion. 

Rebellions


Take two cards and choose one of the shown territories to rebel in. 
Roll 3d6. That many troops in the chosen territory are replaced with your troops. 
If there are any enemy troops remaining in the territory, you must fight them, but if you win, you can continue your turn as normal. If you lose,  your rebellion failed but you can try again on your next turn.


It leads to a more chaotic game but it's a LOT more fun that one or two people sitting around doing nothing for hours.