Generalize the code to work with Summerize/Summary.
dt = current data table();
Summarize( g = By(:Column 0, :Column 1 ) );
For( i = 1, i <= N Items( g[1] ), i++,
r = dt << Get Rows Where( dt:Column 0 == g[1][i] & dt:Column 1 == g[2][i]);
dt:Column 3[r] = Concat Items( dt:Column 2[r], "_" );
);
nt = dt << Summary( Group( :Column 0, :Column 1, :Column 3 ) );