Seems to be some confusion on this as some might assume from the description you need to fill a squad consisting of people in your actual clan. You do not need to have all 4 people from the same clan in the same match.
*Updated Thanks to Maka91: doesnt work on join in progress, very important information.
All you need to do is be in a clan. You can create one by acquiring Grace Larsons' clan services via progressing in the story. Then once you've finished creating press the menu button, then LB until you reach conflict. You can matchmake from there. The achievement will pop as soon as the match starts.