Whether playing with friends or net-friends , the assurance that every member has the same game difficulty is very important for the game experience.Which can prevent the situation that I die under a single shoot from the enemy while my teammate can cross the battlefield as the Superman.LOL
In my opinion, It can be solved by the following ways:
a) Every member's difficulty adjust automatically to the same as the room's owner. and make sure every member know the exactly difficulty they are under.
b) Maybe decide game's difficulty by voting is a very efficient way with less problems.
c) Also, choose difficulty before matching online is a solution. while difficuly can be choosed before starting a inviting-friends game.

Above words is just my own idea and suggestion,I hope to see those changes in the future update patches, and welcome everybody's opinion.