Even though it’s likely that you strained your groin by over-stretching it in the first place, gentle stretching (though, counterintuitive perhaps) is the way to go! You’ve got to start someplace in your mission to get back to normal… Be sure that your leg is supported while gently stretching so that no muscles of your leg feel like they have to contract. Just very relaxed and gentle. It’ll tighten back up, but then loosen it up again. Keep it from getting stiff. It’ll eventually loosen up if you keep the circulation going through it frequently and gently throughout the day. Little by little you can ramp up the intensity of the stretch as you feel the pain decreasing over the days and weeks ahead. When the pain is minimal, then it’s time to start strengthening again. In the meantime, take smaller steps so that you don’t aggravate it.