Replace values in a given column (column is comma separated id's) with lookupvalue from a query

Ian,

I'd use Index to avoid possible errors in enumeration and extract list to combine from GroupByIndex[Count] column. You don't need filter [email] then. Like this and attached

let
    Source = Excel.CurrentWorkbook(){[Name="WorkTracker"]}[Content],
    RemoveOtherColumns = Table.SelectColumns(Source,{"ID", "Team Members"}),
    AddIndex = Table.AddIndexColumn(RemoveOtherColumns, "Index", 0, 1),
    TextsToLists = Table.AddColumn(AddIndex,"Custom", each Text.SplitAny([Team Members],",")),
    ListsToRows = Table.ExpandListColumn(TextsToLists, "Custom"),
    IDasNumber = Table.TransformColumnTypes(ListsToRows,{{"Custom", Int64.Type}}),
    MergeWithLookupTable = Table.NestedJoin(IDasNumber,{"Custom"},
        UserInfo,{"user"},"UserInfo",JoinKind.LeftOuter),
    GetValues = Table.ExpandTableColumn(MergeWithLookupTable, "UserInfo", {"email"}, {"email"}),
    GroupByIndex = Table.Group(GetValues, {"Index"}, {{"Count", each _, type table}}),
    ValuesToCSV = Table.AddColumn(GroupByIndex, "AT Team Members",
        each Text.Combine(GroupByIndex[Count]{[Index]}[email],",")),
    ExtractID = Table.AddColumn(ValuesToCSV, "ID", each [Count][ID]{0}),
    RemoveUnused = Table.SelectColumns(ExtractID,{"ID", "AT Team Members"})
in
    RemoveUnused