how can You can assign the 'Lightning Experience User' permission to a set of users
The "Lightning Experience User" permission can be assigned either through a profile or a permission set.
Profile - gives the permission based on the profile - all users with the profile have the permission
Setup > Home > Users > Profiles > The Profile's Name > Edit > Administrative Permissions > Check Lightning Experience User > Save
Permission Set - gives the permission based on user - select exactly which individuals get the permission
Setup > Home > Users > Permission Sets > New > Name the permission set > Save > System Permissions > Edit > Lightning Experience User > Save > Manage Assignments > Add Assignments > Select the users > Assign > Done
Once you've assigned the permissions, make sure you activate the lightning experience for the org
Setup > Lightning Experience > Skip to the end > Enable > Finish Enabling Lightning Experience