They often just use each other, but I know my local medical school hires actors to be patients. I don't know if they are examined or just interviewed.
I've volunteered for medical studies before; since they were not "real" they were a bit more fun. One I did not do was a study for a blood pressure medication that needed twice-daily testicular exams. I imagine some folks here would jump at that, but I think I would have suffered severe and lasting damage from embarrassment. Plus, I was worried about taking any drug that might do damage; I limited myself to "proof" studies on equipment or drugs that we knew were safe.
I did a feeding study just before I was diagnosed with Crohn's disease. In retrospect, I'm sure I messed up their data a lot and they probably threw me into the reject pile. We were trying a new high-fat enteral feeding solution developed for AIDS patients, and we were the healthy college students comparing normal diets against two formulas.
I got really sick from the high fat and had a lot of accidents. We also had do a couple exams, daily weights, and blood draws every few days. A lot of urine and stool collections which were not a great time as it's seriously inconvient to haul all the collection stuff around campus all day. Just before a stool collection would give us red or blue dye capsules to time our stool, and so it came out bright red-orange or green. I ended up with green stains on my underpants when I did not make it in time.
It was kind-of fun going through it in a group, where you could talk about what they just did to you in the other room without being rude, 'cause all of us were there for the money (and a month with no food bills helped a lot as well).
Another study I did they had me change into gowns and mapped my brain under MEG...did it every week or two for a few months. When they upgraded their gear, I had to stop as without my glasses on I had to squint to read the screen I was supposed to follow, and my head muscles were too "noisy". But that was kinda fun. They were going to give us all DVDs of our brain working, but they screwed up and lost all the data (and I think their funding). I liked that because I got to go into this big outpatient lab and strip down to a gown, put on the disposable footie socks, go through all the medical stuff but I was not a real patient, it was all sort of pretend.
I would walk down the hall in a gown with a tech holding a giant bundle of EEG leads attached to my head and people would stare at me. It's so different when it's not real!
I was a University student for 12 years, so a lot of opportunities.