your formula is fine, but I think it is in the wrong place. If it is in the group footer that you want to see EFGH in, it will only be run once...on H.
if you place it in the Detail Section, it will be run multiple times, and you will need another formula that is just:
global stringvar Letters
to be in the group footer.
ps If you want to hide the Letters from displaying as the report runs, there are many options, but the one that I use is to place "" at the end of the formula. Crystal always prints the last thing in the formula...
HTH