How to build a TemplateBox with dynamic length GridBox?

Thanks to jkuczm for an idea: https://mathematica.stackexchange.com/a/159407/5478:

MakeBoxes[ActuarialSurvivalProbability[x_, t_], StandardForm] := 
 With[{row = Table[Slot[i], {i, 2, Length[x] + 1}]} (*Key point,
  row of slots generated*), 
  TemplateBox[
   Prepend[MakeBoxes /@ Unevaluated@x, 
    MakeBoxes[t]] , 
   "ActuarialSurvivalProbability",
   InterpretationFunction -> (RowBox[{"ActuarialSurvivalProbability", 
        "[", RowBox[{RowBox[{"{", RowBox[Riffle[{##2}, ","]], "}"}], 
          ",", #}], "]"}] &), 
   DisplayFunction :> (RowBox[{SubscriptBox["", 
         TagBox[GridBox[{{#}}, GridBoxDividers -> {}], "Grid"]], 
        SubscriptBox["p", 
         RowBox[{"", 
           TagBox[GridBox[{row}, 
             GridBoxDividers -> {"ColumnsIndexed" -> {}, 
               "RowsIndexed" -> {1 -> Thickness[1]}}], "Grid"]}]]}] &),

   Tooltip -> 
    "probability that atleast one of live(s) aged " <> ToString[x] <> 
     " survives for " <> ToString[t] <> " year(s)"]]

You can use TemplateSlotSequence for variable length TemplateBox arguments:

MakeBoxes[ActuarialSurvivalProbability[x_, t_], StandardForm] := TemplateBox[
    Prepend[MakeBoxes[t]] @ BoxForm`ListMakeBoxes[x, StandardForm],
    "ActuarialSurvivalProbability",
    InterpretationFunction -> (
        RowBox[{
            "ActuarialSurvivalProbability", 
            "[", 
            RowBox[{
                RowBox[{"{", RowBox[{TemplateSlotSequence[2, ","]}], "}"}],
                ",",
                #1
            }],
            "]"
        }]&
    ),
    DisplayFunction -> (
        RowBox[{
            SubscriptBox["", #1],
            SubscriptBox[
                "p",
                RowBox[{
                    "",
                    GridBox[
                        {{TemplateSlotSequence[2]}},
                        GridBoxDividers->{"RowsIndexed"->{1->Thickness[1]}}
                    ]
                }]
            ]
        }]&
    ),
    Tooltip->"probability that at least one of live(s) aged "<>ToString[x]<>
        " survives for "<>ToString[t]<>" year(s)"
]

Then:

ActuarialSurvivalProbability[{x, y, z}, t]

The advantage of using a TemplateSlotSequence solution is that the DisplayFunction/InterpretationFunction can be defined in a stylesheet.

A brief comment on TemplateSlotSequence

I think since the pure functions used in DisplayFunction and InterpretationFunction do not evaluate in the Wolfram Language kernel, TemplateSlotSequence was introduced as a way to enhance the creation of box structures. When the front end constructs the box expression using the DisplayFunction/InterpretationFunction, TemplateSlotSequence objects are interpreted as:

TemplateSlotSequence[1] -> Sequence[#1, #2, ...]
TemplateSlotSequence[2, ","] -> Sequence[#2, ",", #3, ",", ...]
TemplateSlotSequence[{3, 5}, " "] -> Sequence[#3, " ", #4, " ", #5]

Note that the above is not documented, and as with all undocumented functionality, it is subject to change.