We have a project to do with them but I don't have the time and am not sure if this email is appropriate for them. What do you think? I would greatly appreciate your advice. 

Here is what I am thinking of sending them.

*Hello guys,  
Hope you having a good weekend  
I’m writing to you to let you know that unfortunately and for personal reasons, I have to drop this class. I have sent an email to the professor, so he should be aware of it very soon. 
I hope you understand, and I wish you guys all the best on the project. 
Sincerely,*