You could use the three-formula method, if you are willing to use the group footer instead:
//{@reset} to be placed in the group header:
whileprintingrecords;
stringvar type := "";
//{@accum} to be placed in the detail section:
whileprintingrecords;
stringvar type := type + {table.typevalue} + " ";
//{@display} to be placed in the group footer:
whileprintingrecords;
stringvar type;
Then suppress the detail section and drag the groupname into the group footer.
Or, if you must display this in the group header, you could insert a crosstab, using {table.typevalue} as the column and for the summary field, use "NthMostFrequent of {table.typevalue}". You would need to format it to suppress row totals (customize style->suppress row totals), and also you would want to suppress the column headings and the total label, and eliminate the grid (customize style->format grid lines->uncheck show grid). You could then resize the label cells to minimize them.
The final option is to create a subreport linked on customerID that uses the three formula method above, and place this in the group header.
-LB