Is it worth joining a guided tour?
Maybe yes, maybe no.
Verdun is a French army battle site, so many of the bigger tours are in French. Great if you speak it, not so great if not.
My suggestion would be to start by going to see the film shown at the Ossary, which runs near enough every half an hour. They show versions in French, English and German, so depending on when you arrive, you may have a wait but it's probably worth it. It will give you a good idea of what the battle was all about and how the place still has a special place in the French psyche to this day. You can do enough simple research on the internet to create your own tour without too much difficulty.
The most common mistake bods make is to assume that battles take place in quite a small area; they don't, at least not in the 20th century. Watch the film to get a scale of the affair, arm yourself with a half decent map, a simple guide book (they are cheap) or make your own tour. You'll have fun and could well end up seeing quite a lot.