When performing the Copy production data to Sandbox function, provide site admins the option to copy all group members or not.

At the moment according to the documentation, the group membership won't be copied. 

• https://support.atlassian.com/organization-administration/docs/manage-product-sandboxes/#Copy-data-to-a-sandbox

Product access settings (you’ll need to add users to groups for permissions to apply)

I would like to preserve the membership. Using groups vs per-user permissions is one of the cornerstones of users management best practices, and redoing all group membership for several hundred moreover thousand users is quite confusing.