I couldn’t find it. i ended up creating a custom field for contact details and using that to filter people. But it’s not ideal as the field can only assign one value to a contact at a time. If I have contacts related to multiple projects, I usually have to duplicate the contact.
I don’t understand why grouping contacts isn’t a feature. But I’m more concerned that the developers don’t see the need for such a feature. Seems like a major oversight to me as it was one of the first things I needed.