Ahh, looks perfect. Knew I couldn't be the first one with this need :)
I use Auldlangsyne, which allows you to add a custom note to anyone in your guild / friend / ignore list.
Also adds an icon to fubar / minimap then when moused over gives guild / friends summary including a quick view of all custom notes.
So the easiest way would be to add them to friends list, with note a note like PUG: 2v2