Sorry for the unclarity.
Yes, you're right: obviously it would be a misnomer to call it a "shareholders' agreement" if it includes non-shareholders.
I was aiming at the principle of whether anyone else can be brought into the contract* -- along the lines of an unincorporated association, where any class of member is possible, on any terms you like.
Eg, for a case where non-shareholder directors aren't employed, but you want to have the sort of nuanced, private agreement that The Resolver mentions. You might want the directors liable in contract to the shareholders as such, and not liable to the company for breach of duty.
*ie, the contract that in most cases includes only shareholders but in the one I envisage, also includes non-shareholder directors