Since DH and I have had one, I'd like to put my 2 cents in.
Instead of a stag and bridal shower our Jack and Jill was a shower for the two of us where guys, girls, family, friends anyone we would have invited to my shower or his stag could come. No one had to come as a couple if they didn't have a significant other. No one had to buy tickets to get in either. Our parents paid for the hall, dinner, open bar, and decorations. We had gambling, a raffle, a money wheel and a DJ. There were over 200 people and to the best of my knowledge everyone had a blast. People brought gifts, but not cause they were told to, just cause that's what they would have done had I had a shower or DH a stag. It's just a nice way for everyone to get together!