Boosting a teammate is easy! Go to the "Team" page and click on "Booster" for the teammate you want to boost. You can also boost the teammate you want directly by going to his or her profile from the same page.
Important: if you boost a teammate, the boost will be valid for 24 hours only. Similarly, if you receive a boost from a teammate, it will also count for 24 hours. Work with your team to place your boosts as effectively as possible ;)