two options
1. convert the text field to a date field and do as i suggested before. The ct header can be set to look like anything you want including the MMYY format with ease.
2. just extract the YYMM fields from the text and do your grouping on that.
I prefer option one as it gives you a lot more flexibility in using the datefield type for select criteria and grouping and can be easily changed for display purposed into numerous formats.
here is one way toconvert your string to a date
date(mid(field,5,2) + '/' + mid(field,7,2)+'/'+left(field,4))