The explain plan will now contain a GROUP stage in the winninPlan part of the queryPlanner section of the explain plan. For an example see,

"winningPlan" : {
"queryPlan" : {
"stage" : "GROUP", <--- new node.
"planNodeId" : 2,
"inputStage" : {
"stage" : "COLLSCAN",
"planNodeId" : 1,
"filter" : {

},
"direction" : "forward"
}
},
"slotBasedPlan" :

",
"stages" : "[2] mkbson s11 [_id = s7, s = s10] true false \n[2] project [s10 = doubleDoubleSumFinalize (s9)] \n[2] group [s7] [s9 = aggDoubleDoubleSum (s8)] \n[2] project [s8 = getField (s4, \"x\")] \n[2] project [s7 = fillEmpty (s6, null)] \n[2] project [s6 = getField (s4, \"_idMod10\")] \n[1] scan s4 s5 none none none none [] @\"9d840c15-b98e-4daf-a0d5-e79fdfa85311\" true false "
}
},