Have a look at the "Your IPK Groups Only" as an example. This is a search field (non-physical) but is currently defined as a dropdown. Basically you'll need a new record in SU_FIELD table (remember to use su_number_alloc to get the next ref), but you want to mimic the "Default IPK Group" field definitition (so it is displayed as a QD) while still being treated as a non physical search field. I susgest giving it a field name of IPK_GROUP_MEMBER or something along that line (has to be unique).
Then edit the search-sql-person.xsl (or create a search-sql-person-custom.xsl) and make a copy of the DEFAULT_IP_GROUP block and change the fieldID to IPK_GROUP_MEMBER. Finally change the expression within so that the exists is looking up the view SV_HC_TO_GROUP. Have a look at the IPK_GROUPS block for an example. Once the expression is defined correctly, you should have a working search criteria. Then it's just a matter of repeating it for workflow group.