Is it okay for a reviewer to share an under review manuscript with their colleagues?
Do not share this manuscript.

It is one thing for you to solicit input, under an agreement of confidentiality, when you recognize there is another expert who can help with the review.

It is totally different when someone demands that you share it, not because you are getting help with the review, but for their own edification.  The manuscript is confidential.  Keep it that way!