The book goes into a lot of detail, and a lot of depth. Reading a chapter from front to back, and marking with a highlighter is, for most people, not the best way to learn biology. If you feel up to it, reading before you come to lecture can be a great way to familiarize yourself with the material. But if you hear the lecture first and then go back and read the book, you'll have a better idea which details your instructor considers most important. Study alone, then study together.
Most people study best if they combine solo work with group work.
But beware of too much group time - you'll be working solo on exam day. Effort is not the same as results. Unfortunately, sometimes effort just doesn't pay off. You don't get graded on how many hours you study - you get graded on what you learn. If you're putting in the effort and it doesn't seem to be paying off, ask for help.