In the future, you might want to consider watermarking your photos so they can't be stolen as easily.

I see a lot of pictures that are done that way and I don't think it detracts from them when done properly.

I've a hunch that there is little practical value in obtaining a lawyer for a situation like this.

(still not a lawyer)

There is a site posting ads of dozens of girls and using their pictures without their permission.  Short of banding together and hiring an attorney, what is the best path to take?

This is something that will probably require an attorney.

Technically, the people using the pictures are violating Copyright law.  But, in order to sue them for using the pictures, you would need to get their real names and addresses.  Getting that information from the site will probably be impossible without an attorney.

The attorney will probably:
1.  Send a "cease and desist" letter to the owner.  This is ignored 99% of the time but might be a necessary first step.  
2.  If the violation continues will file a lawsuit in court against John Does, Jane Does, ABC Corporation and other placeholder names which will allow them to get into court and ask the judge for an order to force the site to reveal the information about the violators.
3.  Then those people will be sent Certified letters and, if they don't comply, the suit will proceeed.

At least this is the process that I had to go through to deal with websites that have stolen my copyrighted material.

But, in my case, the material that was stolen was wholly legitimate.  If the pictures being used are those of escorts, and they're being used in ads for escorts, it's possible the judge will simply through the case out because of the illegality of escorting.  Yes, they can claim they are models, etc. but I think the average judge will make the assumption that there's something else going on.

Now the ladies can send individual "cease and desist" letters to the website but the people running the site will probably use the letter for kindling.

(not a lawyer; I've just dealt with copyright issues numerous times.)

There was a thread about this a while ago:

http://www.theeroticreview.com/discussion_boards/viewmsg.asp?MessageID=400659&boardID=12&page=

This is probably one of those situations.