HI Markus,
In the table settings for the "Decision table" expression in the BRF+ rule, add the "User Group" object from the "GRAC_S_REQUEST_RULE_LINE" structure to your condition column.
This is the SU01 usergroup you want to utilise based on the entries within "User System Details" tab in the Request screen.
See the screenshots below from BRF+
the one you require to select is highlighted
Hope that makes things clear for you