Certain skills fall into the category of synergy skills. These skills have an effect when cast, but in addition, they put a large red circle on the ground (which I wish was a different color, because it looks like an enemy area attack circle, which I am conditioned to avoid ). A group member who's inside the circle can press X to activate whatever the synergy effect is. So you have to pay attention to when they're cast, and then get inside the radius.
Beyond that, they all work differently; depends on the skill. Read the skill descriptions Synergies are the ones that contain the text "Allies may activate the X synergy". http://eldersouls.com/elder-scrolls-online/skills
ALL the Undaunted guild skills are synergy skills, so that's a good place to start; but there are plenty of others scattered around the trees, for instance, DK ultimate "Dragonknight Standard", Sorc ultimate "Storm Atronach", and Templar ultimate "Nova".