Mailmerge query criteria

[PEASLET]

In a mailmerge query that I'm using there is a field called 'subgroup'. Some of the recipients are in more than one subgroup and so they are getting a letter for each subgroup that they belong to. I would like to send them one letter but with all the subgroups that they belong to listed (in the address block). Does anyone have any idea what criteria would I put in the subgroup field to make it so?!
Thanks
Theresa