An individual who adds to the overall appearance or dynamics of a group but has no interesting qualities on their own. Similar to the herb parsley, which is used in many dishes as a garnish; these individuals "garnish" a group interaction (oftentimes in a party or entourage) but have no flavor or personality by themselves.
L: Why would you invite Luis to the party? He's so boring!
D: Yeah, but the guest list is too exciting. We need to throw some parsley in there.