Table.Group problem

Hi newhopepdx, apologies for the delay. Please try the revised solution below:

let
    Source = YourTable,
    AddFullName = Table.AddColumn(Source, "FullName", each [FirstName] & " " & [LastName]),

    Grouped = Table.Group(
        AddFullName,
        {"GrpId"},
        {
            {"GroupData", each _, type table [GrpId=number, PersonId=number, GrpLeader=number, FirstName=text, LastName=text, FullName=text]}
        }
    ),

    AddGroupName = Table.AddColumn(Grouped, "GroupName", each
        let
            groupTable = [GroupData],
            leaderId = Record.Field(Table.First(groupTable), "GrpLeader"),
            leaderRow = Table.SelectRows(groupTable, each [PersonId] = leaderId){0},
            leaderFirst = Text.From(leaderRow[FirstName]),
            leaderLast = Text.From(leaderRow[LastName]),
            otherMembers = Table.SelectRows(groupTable, each [PersonId] <> leaderId),
            otherFirsts = List.Transform(Table.Column(otherMembers, "FirstName"), Text.From),
            lastNames = List.Distinct(List.Transform(Table.Column(groupTable, "LastName"), Text.From)),

            groupName =
                if List.Count(lastNames) = 1 then
                    Text.Combine({leaderFirst} & otherFirsts, " ") & " " & lastNames{0}
                else
                    Text.Combine(
                        List.Transform(
                            List.Combine({{leaderRow[FullName]}, Table.Column(otherMembers, "FullName")}),
                            Text.From
                        ),
                        " & "
                    )
        in
            groupName
    ),

    Final = Table.SelectColumns(AddGroupName, {"GrpId", "GroupName"})
in
    Final