For exercise it doesn't matter what sort of bike you ride as long as it fits you and is comfortable.

If your friends are going to be pushing it on their rides, you won't be able to keep up on a hybrid, but even with a road bike it will take you a while to get to up to speed. If its a relaxed pace ride, you'll do ok on a hybrid, except that I find flatbars uncomfortable for long trips.

I suggest getting an inexpensive hybrid and later if you really get into riding, maybe a road bike.