At the very beginning, I think you'll have to re-model the data.
let
Source = Table.FromRows({{1111 ,"5553", "6112"},
{1111 ,"2118", "5553"},
{1123 ,"5559", "6088"},
{1828 ,"6169", "3509;5375"},
{2494 ,"1672;6057", "4062"},
{2961 ,"5649", "5648"},
{2500 ,"","4726"},
{2653 ,"6112", "5026"},
{1111 ,"5553;6112", "1433"}},
{"PortfolioID", "SellerID" ,"BuyerID" }
),
#"Added Index" = Table.AddIndexColumn(Source, "Index", 0, 1),
#"Split Column by Delimiter" = Table.SplitColumn(#"Added Index","SellerID",Splitter.SplitTextByDelimiter(";", QuoteStyle.Csv),{"SellerID.1", "SellerID.2"}),
#"Split Column by Delimiter1" = Table.SplitColumn(#"Split Column by Delimiter","BuyerID",Splitter.SplitTextByDelimiter(";", QuoteStyle.Csv),{"BuyerID.1", "BuyerID.2"}),
#"Unpivoted Columns" = Table.UnpivotOtherColumns(#"Split Column by Delimiter1", {"PortfolioID", "SellerID.1", "SellerID.2", "Index"}, "Attribute", "Value"),
#"Unpivoted Columns1" = Table.UnpivotOtherColumns(#"Unpivoted Columns", {"PortfolioID", "Index", "Attribute", "Value"}, "Attribute.1", "Value.1"),
#"Renamed Columns" = Table.RenameColumns(#"Unpivoted Columns1",{{"Value.1", "SellerID"}, {"Value", "BuyerID"}}),
#"Removed Columns" = Table.RemoveColumns(#"Renamed Columns",{"Attribute.1", "Attribute", "Index"}),
#"Reordered Columns" = Table.ReorderColumns(#"Removed Columns",{"PortfolioID", "SellerID", "BuyerID"})
in
#"Reordered Columns"
Then a calculated table as
Table =
SUMMARIZE (
FILTER (
're-model table',
IF (
CONTAINS (
FILTER (
CROSSJOIN (
SELECTCOLUMNS (
're-model table',
"PortfolioID", 're-model table'[PortfolioID],
"buyerid", 're-model table'[BuyerID],
"sellerid", 're-model table'[SellerID]
),
SELECTCOLUMNS (
're-model table',
"PortfolioID2", 're-model table'[PortfolioID],
"buyerid2", 're-model table'[BuyerID],
"sellerid2", 're-model table'[SellerID]
)
),
[PortfolioID] = [PortfolioID2]&& ( [buyerid] = [sellerid2]
|| [sellerid] = [buyerid2] )
),
[PortfolioID], 're-model table'[PortfolioID]
),
FALSE (),
TRUE ()
)
),
're-model table'[PortfolioID]
)