How to score class participation, correcting for natural outspokenness?
Is "speaking in class" or "communicating about the topic in groups" part of your learning goals for your students? If yes, then you should first communicate this to your students and then think about how to include participation in the grading rubric based on the learning goals you set.

However, if you only want students to participate in the lecture to make your own teaching experience better, I think it is unreasonable to include their participation in their grade.

Put differently: I think that the grading should reflect how well the students achieved the learning goals of the course and nothing else. (This may be different in school, but for a undergraduate and graduate courses, I think this is reasonable.)