It has crossed my mind a few times but I'd probably never seriously go through with it. I have seen agency websites recruitment page in whcih the WG has to fill in her details, attach photos and she will then be invited in for an interview and photoshoot. I stand to be corrected,but I get the impression than any girl (as long as they are reasonably attractive and confident) can sign up/register with an agency, as they are not actually "employed" by the agency, as the girls generally choose to work as many or as little hours as they like- this does obviously not happen in a normal workplace. The interview is probably the pimp explaining to the girl exactly what escorting involves, about the incall location, types of clients etc. The agency just acts as a middleman between client and girl , though they provide an incall flat, and transport. Girls are generally expected to buy condoms, any sex toys, stockings, uniforms and bondage materials like cuffs etc themselves.
Agencies get a really bad name amongst alot of independent girls, but I think good, reliable agencies have their place. I worked for one very briefly before I went indie, although I would never do it again myself, I can see why some WGs do prefer that way of working. Your advertising is taken care of so you don't spend ages every day responding to emails, you don't have to deal with timewasters and so on, and some girls feel it's safer, although I'd say that's very, very debatable.
What Dime describes is pretty much exactly how things were with the agency I worked for when I was there, although it was outcalls only and you could only choose your hours to an extent - you had to work at least 5 nights a week, and if you were on call you were expected to be up all night waiting on jobs and more or less ready, as you had to get to the clients in around half an hour. I understand that's the case more or less everywhere. Interview was the lady who ran the agency explaining hours, fees, set up, what she expected of you and so on (no 'casting couch', although I have heard of that going on). We had to supply all condoms, outfits and so on ourselves.
I've often thought that if you set up an agency, treated the girls right, screened clients properly and gave them the right girl for them and took a fair cut, you could be really successful, although as others have said you have the police and POCA to worry about too. It's not something I'd ever consider doing myself though.