It won’t be required or forced in my opinion. Larger guilds right now already enjoy a variety of advantages in getting groups together and such. So while the new system may encourage players to take part in guilds, it can be avoided if wanted.

I have a pvp guild that is very small (about 10 members). Will it be theoretically possible to earn all the guild perks, even if we dont raid at all?
I can’t say anything is final right now, but the idea (last I heard) is that there would be multiple ways to level the guild – it just may take longer for a smaller one. As expected though, we hope to do some testing on this once things are ready in the beta to get a better feel for what works and what doesn’t.

Small guilds disadvantaged by guild perks?
In my experience the people who like to focus on smaller guilds don’t really care what the other larger guilds are doing as they are happily doing their own thing. It also sounds like you don’t fully understand the guild system and we’ll have more details to help you out when we are farther along in the beta. (Source)

“Balancing” Guild Perks
I’m sure we’ll be playing around with it a fair amount in the beta. We don’t want it to be a mega penalty to lose the rep to where players stay in a guild they don’t like – but at the same time a lot of these changes are meant to encourage guilds as a whole so we don’t want it to be extremely trivial to gain the reputation to where players give no thought to guild hopping (like now). We know this may shake things up for some players but we feel it will be a better result for the game as a whole to have a stronger setup for guilds. (Source)