I was wondering if anyone knew of a good pool to join in the D.C. area for the summer time?

 I'm not much help.  The pools in Montgomery County are incredibly expensive. 

I am going to join the Montgomery Aquatics Center in Rockville, MD, though.  It's owned by the Parks Department, so it's a lot cheaper.  I don't believe they have outdoor pools, though.

Thank you. That is actually a lot of help. I will have to look into the Aquatic Center because I would also like my children to take swimming lessons as well so that they will be ready for next summer

That's your best bet, then.  The swimming classes are very reasonable!  We're going to be doing one this winter, too.

Here's a link to the Montgomery County Parks Department Catalog of Classes:  http://www.parkpass.org/Start/Start.asp?SCheck=208813146&SDT=39678.7032523148

They have tons of classes!