What you do is you follow your team mates and activate your frost Shot, and keep whomever they are attacking slowed, and stop attacking them when they are about to get the kill(although two or three kills early game helps you build faster)

In late game team fights, you want to keep moving and try to keep the entire enemy team slowed, thus giving your team a massive advantage throughout the fight. Use your ult to pick up kills when people are trying to run away, or to engage a team fight