Jellyrolls is not operated by WDW, so I don't believe that you can use room charges, but I could be wrong there.
There is a cover, and last I visited, I believe it was $10 cash. Drinks are reasonable, and they have free popcorn. They don't allow videotaping inside, and they may also not allow photography. Servers are very good, and the entertainment is fantastic.
I've been going for years and have seen some of the same folks up singing / playing, and they're really talented and engage the crowd. I strongly recommend "Take Me Home Country Roads" by John Denver, "Red Solo Cup" by Toby Keith, "My Ding-A-Ling" by Chuck Berry, and "Paradise by the Dashboard Light" by Meat Loaf.
Have fun!