grouping of formula field in crystal report

i have a formula field called rawmaterial in which i have written the following coding.

WhilePrintingRecords;

Shared StringVar MaterialNo;

StringVar ItemNoMain;

if {V_ARINVT_AVAIL_PROJECTED_DTL.SOURCE}="PO" THEN

ItemNoMain:= {ARINVT.ITEMNO}

ELSE

ItemNoMain:=MaterialNo

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the report displaying ItemNoMain correctly, but my problem is i want to group the itemNoMain data