ZapMcRaygunn: While I would recommend joining just because it will make the story more enjoyable, there is a cheat/workaround that will let you finish the game. This "cheat" actually uses an in-game item. I'm not going to tell you how to do it because I don't want to leave spoilers, but let's just say a certain flying means of transportation that is actually on the game map will give you the tool you need to win the game without joining the fellowship. That said, it is still better to join just to get the full story.

One warning: If you should meet a character named Tseramed in your travels, DO NOT tell him you are a member of the Fellowship. doing so will start a fight and prevent you from gaining a valuable party member. In fact, try to find him before you join the Fellowship because once he is in your party he will not react in any way when you join. He seems to understand your motives and that you are doing so to spy on the organization rather than to support it.

HenriqueVT: I met him just after i joined the fellowship. I was still able to recruit him, though. Is this going to make him turn on me later?

Garran: You do not actually need to join the Fellowship in order to complete the game; you'll miss out on a few plot developments (as part of a branch that eventually leads to a dead end), but that is all. The item mentioned by Zap isn't actually a "cheat" either - by design there are a couple of ways around the problem in question.