The "Share" option is visible on Account and Contact (and possibly lots of other entities but for me now it's only relevant with these three).
It is not however visible on Opportunity. We have made no customizations to the ribbon and I'm sysadmin.
On the other hand, when I look at any "Opportunity grid" and select an opportunity record the "Share" button becomes visible...
So, why isn't the button visible on the Opportunity Form? Any and all ideas are appreciated.