Add index or rank number based on groups in Power Query

Hi cathoms 

You will need to use the Group By multiple times and create an index column within one of the tables created by grouping.

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WMjRT0lEyMNE3NNQ3MjAyVTA0sTIwAAoZmir4ZuaVlqQWK8XqRCsZG6EpM7YyMcVUZm5OkTJjA6KUmRAwDbsXyFdmiMVthmZoAWeJ1TRcyog0zYSgaYZYgheLMmNi3GZoiumHWAA=", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [PROV_ID = _t, SLOT_BEGIN_TIME = _t, SLOT_LENGTH_MINUTES = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"PROV_ID", Int64.Type}, {"SLOT_BEGIN_TIME", type datetime}, {"SLOT_LENGTH_MINUTES", type text}}),
    #"Grouped Rows" = Table.Group(#"Changed Type", {"PROV_ID", "SLOT_BEGIN_TIME"}, {{"Group", each _, type table [PROV_ID=nullable number, SLOT_BEGIN_TIME=nullable datetime, SLOT_LENGTH_MINUTES=nullable text]}}),
    #"Grouped Rows1" = Table.Group(#"Grouped Rows", {"PROV_ID"}, {{"Group", each _, type table [PROV_ID=nullable number, SLOT_BEGIN_TIME=nullable datetime, Group=table]}}),
    #"Added Custom" = Table.AddColumn(#"Grouped Rows1", "Group With Index", each Table.AddIndexColumn([Group], "Index", 1, 1)),
    #"Removed Columns" = Table.RemoveColumns(#"Added Custom",{"Group"}),
    #"Expanded Group With Index" = Table.ExpandTableColumn(#"Removed Columns", "Group With Index", {"SLOT_BEGIN_TIME", "Group", "Index"}, {"SLOT_BEGIN_TIME", "Group", "Index"}),
    #"Expanded Group" = Table.ExpandTableColumn(#"Expanded Group With Index", "Group", {"SLOT_LENGTH_MINUTES"}, {"SLOT_LENGTH_MINUTES"}),
    #"Changed Type1" = Table.TransformColumnTypes(#"Expanded Group",{{"SLOT_BEGIN_TIME", type datetime}, {"SLOT_LENGTH_MINUTES", type text}, {"Index", Int64.Type}})
in
    #"Changed Type1"