Relating back to the parent table

[FMDuck]

I'm a little 7 brain twisted!

I have 2 tables: People and Groups - I've placed a Groups portal (Cartesian) on the People layout. In groups I have GroupName, PeopleID and cPeopleID. PeopleID is a comma delimited list of group members and cPeopleID is a calc field that shows the person viewed in the parent record is a group member (shown by a check box).

PeopleID populated normally via a script but I can't get cPeopleID to calculate as desired. I'm using PatternCount(?;?) and tried a variety of options with and without new TOs but just can't visualize it and am not getting lucky!

Thanks in advance.