Prepare charcoal fire or outdoor gas grill for covered direct grilling over medium heat.
Press shallot through garlic press into a cup; stir in tarragon, mustard, pepper, and salt. Spread tarragon mixture all over tenderloins.
Place tenderloins on hot grill rack. Cover, grill and cook, turning occasionally, until the meat thermometer inserted in center of tenderloins reaches 155 degrees F (18 to 20 minutes). When done, let pork stand for 5 minutes, as internal temperature will rise to 160 degrees F.
After pork has cooked for 15 minutes, add the bunch of grapes to same grill rack and cook for approximately 4 to 5 minutes. Turn occasionally.
Place tenderloins on cutting board. Place grapes on large platter, with kitchen shears, cut into 8 clusters. To serve, thinly slice tenderloins and transfer grapes to platter.