I dunno... I think people often overthink this stuff. Unless you're looking for something really weird, most companies make a decent selection at your pricepoint, so as long as you try a few different bikes - like three, maybe, unless you're still feeling out what type of bike you want - and a couple different sizes in at least some of them, you're probably fine buying your favorite. Giant, Cannondale and Trek are pretty dominant, so there should be some available near you. Other brands are definitely worth a look too, and may stretch your dollar further, but may have smaller selections. It's also worth phoning around and asking about consignment bikes. You can sometimes get a really good deal that way.
I ride a Specialized myself - a Hardrock that gets more and more ridiculous with each new season and team deal. It's been a solid frame, and I like it that many of the fittings on Specialized bikes are quite standard - the bottom bracket and headset on their bikes, at least at your pricepoint, are of the most common types, making replacement very, very easy and generally cheaper, or nicer, depending on what you want to do. Your number most likely puts you on a Rockhopper, although in a '10 model, if you can find one, you might also look at a FSR XC or the base-model Stumpjumper hardtail. The Rockhopper Expert 29er has a significantly better fork and nicer spec than the Comp 29er, if you can stretch your budget a little. (Of course, if you stick with the Comp, you'll still have some money left over for some riding clothes, helmet, gloves, etc. etc.)
I ride alone, with teammates or with friends, depending. I think it's more fun with other people, and I get a better workout with most of my teammates than I would on my own. They both have their pros and cons. I think that starting out, I learned a lot, both about riding and about where to find trails, riding with the friend who got me into it. See if you have a club in your area - a lot of larger cities have them, and it's a good way to get to know some people who ride.