// open text file as a table
IFeatureWorkspace featureWorkspace = (IFeatureWorkspace)workspace;
ITable table featureWorkspace.OpenTable("potshots.txt");
IStandaloneTable pStTab = new StandaloneTable();
pStTab.Table = table;
Also, having the table in the map's table collection isn't necessary in order to perform a table join so if that's the only reason you're adding it then you may want to skip it altogether.