I spent a summer working at the Sylvan Lake Lodge, and never had a problem hooking up with people at the Sylvan Lake parking lot near the General Store. Shouldn't be hard to find people at Rushmore either.
The climber's campground used to be Breezy Point near Rushmore, but I hear its been turned into a day use picnic area - which is certainly a shame. There is a ridiculous amount of climbing near the Sylvan Lake campground.
If you want to climb around Boulder - check the partner finder on climbingboulder.com and also the climbingboulder-discuss mailing list - i've met several great partners through both of those. i will probably be in town the week after next, so i may be available to climb then as well if you are still around.
For Needles guidebooks, Touch the Sky is really the only option right now(though apparently Larry Schaeffer is working on a new one - and probably nobody knows the area better than him). You can get updates at Granite Sports(Sue is a good friend - say hi for me). Vern Phinney wrote an excellent guidebook to Rushmore, which has the best easy sport climbing in the world - be sure to check that place out.
The bouldering and hiking in the Needles is superb. Hike up Harney Peak from Elkhorn or Horsethief Lake. The bouldering at Photographer's Peak is full of classic moderates, and the Campground Boulder has great harder stuff. There is a sandstone area kind of like Horsetooth near Rapid City called the Rock Maze - check out climbingblackhills.com or the routes database on rockclimbing.com for information as there is no guidebook. I love the place, though I'm biased since I put up a lot of the problems
The Badlands just east of the hills are well worth going to for a day of hiking - really amazing place.