I went to law school in the Paper Chase years, where study groups were common. I hated studying with others. I found it inefficient and a waste of time. The idea was that you would do better challenged by a group but I just didn't roll that way.
My preference was to study alone late in the evening or early in the morning and then hang out with friends B.S'ing during my free time. Maybe you could try to find other ways to socialize with A and gradually phase out of the group study by saying you already prepared and have to get some bike riding in.
Trek Madone 4.7 WSD
Cannondale Quick4
1969 Schwinn Collegiate, original owner
Terry Classic
Richard Feynman: “The first principle is that you must not fool yourself and you are the easiest person to fool.”