We have found out from the Metroid Prime: Federation Force mission briefing video that the game does not scale the difficulty if you’re playing with less than 4 players. This means that it’s going to be a considerable challenge if you can’t find enough people.
As I mentioned earlier, I think four players playing together locally is the most fun way to play this game. But if you can’t round up others to play with, you do have the option of going online to find allies. Additionally you can play singleplayer. There will be no handicap on your enemies strength though, so it will be quite the difficult task. If you’re an expert player, consider this a challenge from us here on the development team and try going it alone.