Brugger + Thomet makes a picatinney rail fore end for the Benelli M1 and M4/M1014. They are expensive, about $350, but it's a great unit and will allow you to mount the grip or accessories you want.
Aside from that, Benellis are not popular guns to be using picatinney rails on. That's why you don't really have a great deal to choose from regarding picatinney rails, though you think you would with how popular the guns are. Unless you make specific modifications to the gun to compensate for the effects of added weight, mounted accessories cause a corresponding decrease in cycling reliability on Benellis. As the weight of the gun goes up with add-ons, the amount of energy needed to cycle it goes up too. This decrease in reliability won't be noticed when using already sufficiently powerful loads, but whatever the minimum load is that your gun will cycle reliably (most people don't test this), that load will suddenly give you trouble when optics, flash lights, side-saddles and anything else is added.