I have used Tiffany Towncar to RPR, PBH and Disney hotels. I was happy with them. Some other people have been not-so-happy; check out the threads on the Transportation Board.
I believe that for 5 people you need a van rather than a regular towncar. You can check their website. My notes from March say that the price was $70 one-way, $125 round trip. You can get a coupon for $5 off on their website.