Individualize the Group
Group work is a good way to get students of different abilities to work together. Group work helps students with teamwork, brainstorming and critical thinking. When grading, be sure to give a group grade that reflects the groups' performance as a whole. In addition, give each student an individual grade. This way, if one student does all the work and another completely drops the ball, other group members aren't penalized and/or rewarded for their teammates' efforts.