Sure you can...but it certainly makes it easier to find people to help you train with and obtain gear at higher levels. Obviously the game mechanics don't force you to join a guild at any point. Gameplay is not guided by doing missions alone and missions are few and far between at higher levels unless you love to craft. At lower levels there are opportunities to get gear through Sage missions. If you are really good at convincing people to help you then certainly you don't require a guild. Higher level players might also expect to experience some players not willing to help them if their faction doesn't agree with their own. Most seem to be willing to help if they can and if you don't give them a reason to otherwise ignore you.
