They're the only friends we ever had

Strategy: Mulligan for Deft Focus, Team-Building Exercise, and "Think Fast!". Keep the villain confused with Eros and "Think Fast!" to flip down and get those counters up. Once you're at 10 counters you can use a counter for extra stats with Groot's upgrades, but you want to chump block to keep your counters up for "I am Groot" with Overwatch and "I. AM. GROOT!".

You won't need Booster Boots, Endurance, or Down Time, because you'll likely stay at 10 HP the whole game, and counters are easy to bring back up to 10 with enough allies. Also never seem to need Lashing Vines.

"We Are Groot" isn't great solo unless you have multiple allies out, but feels amazing in multiplayer games. The same with Overwatch and "I am Groot" together.

If you're playing with another Guardian player, I'd drop Team-Building Exercise and replace a lot of the Guardians for cheaper Justice allies maybe keeping Gamora and Martinex unless Gam has more synergy with an event player.

With an Avenger player, add multiple Problem Solvers and a Heroic Intuition, go to cheaper Justice allies, and drop The Power in All of Us.