"do we force her to join a club to meet new people and be more social or do we just leave things the way it is?"
"Forcing" her to do anything would not be productive as she will see this as your idea not hers. She has to develop an interest in at least one thing/activity. Why is she not "comfortable" in joining clubs or sports at school is another issue. Is this due to some sort of anxiety (social anxiety, fear of failure), or is she more introverted? When did the behavior start (or has she been behaving like this for as long as you can remember?) The few friends that she has do not have to be her only circle of friends that she does things with. You may find out if she is engaged primarily in her school work because that is one area where she feels safe (good at) and uncertain of herself in other areas? Is she the only child and used to be in the company of adults is another question.
Volunteering may be something to encourage her to look into. She can be both productive/helpful even learn something along the way. If she is studious, you can present this opportunity for further learning and remind her that any sort of experience can come in handy one day when she will need to get a paid job.
There may be times when you would have to entrust her in doing something that lets her interact with others even if it is helping with an yard sale, etc. She may have not yet found out what truly makes her satisfied (school work is only one area) and the more she tries, the more she would learn about herself. You could try to show her that is one way of individuals gaining more self confidence in their abilities and gaining more knowledge in general by interacting with others. Firs, try to have her share what she already is certain of in regard to the reason behind her social isolation with peers.
I am not sure what response is being sought out. The initial question was should you force her to be involved. I caution against that any sort of forceful behavior is to avoided. It is unhealthy and wold not really guarantee her full participation. Since I am not working with the child or the family directly, the information provided online is speculative and suggestive. She may even have traits of avoidant personality disorder.
If she is not interested, she is not interested and no amount of coaxing or otherwise will change her mind. She has to be encouraged to arrive at her own concision as to why it is beneficial to interact with others. You can only teach her that. I will opt out now so any other expert can share their feedback.