Talk with the student about the situation. It's important to understand exactly why he does not want to participate in group work. Whether it's an act of outright defiance or a fear of getting a poor grade if other students do not pull their weight, if you understand why the student is apprehensive, you are more likely able to help resolve the problem. Meet with the student one-on-one to discuss his obstacles or concerns.
Set clear expectations about the student's role. If a student understands exactly what is expected of him and has a sense of responsibility toward the group's success, he may be more motivated to work with his peers.
Consider pairing him with one or two other students, rather than a bigger group of five or six. Students who don't do well in groups because of behavioral issues or social anxiety might work better in pairs.
Give him the option to work independently. Explain that he will be responsible for the entire project or assignment in the same way that a group would be. He may be more motivated to work in a group if he grasps the amount of work involved in doing the project by himself.
Convey that he will not be able to participate in class or will receive a failing grade on the assignment or project if he will not work in a group. In some cases, a student might be trying to push a teacher's limits by refusing to do something. Set clear boundaries and let him make the choice: Either he can work in the group or suffer the negative consequences.
Speak with your school's guidance counselor or principal. It's possible the student has issues at home or emotional problems you're not aware of. Ask for guidance on how to best assist this student if your techniques are not working.