Filter table with another table in Power Query

I finally got a solution working with Table.Contains.

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WilDSUTIEYkelWB0IzwiIneA8YyB2BvMioXJecJ4JELvBeWZA7A3mRQFZ5kAcBudZwE2JgtoXohQbCwA=", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type text) meta [Serialized.Text = true]) in type table [Key = _t, Step = _t, Val = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"Key", type text}, {"Step", Int64.Type}, {"Val", type text}}),
    #"Grouped Rows" = Table.Group(#"Changed Type", {"Key"}, {{"Step", each List.Min([Step]), type number}}),
    Custom1 = Table.SelectRows(#"Changed Type", each Table.Contains(#"Grouped Rows",_,{"Key","Step"}))
in
    Custom1

To get this working, the Table.Contains operates over the grouping step to get the set of keys I want to use as a filter. Then it operates over the _ , which is a record object that is available by running "each" as a comparer function. As my resulting table has more columns than the grouped result, I pass in a list, defined with {}, of the column names that should be used to compare them. If your group step had different column names, you would need to find a way to alias the fields found in the "_" record.