the standard for concatenating multiple rows into 1 line is something like:
formula in the detail section:
shared stringvar x;
x := x + ", " + {table.field};
"" //hides the output
usually the detail section is suppressed.
in the group or the report footer then the following formula is used:
shared stringvar x;
mid(x, 3); // strips off the leading comma and space.
if canGrow is checked, the control should expand in the standard way that CR expands a data column, which is all that can hope for.
HTH