Glenn is looking around for supplies, food, etc to take back to his surviving group, until he sees you getting swarmed

Once you finally found a way out, you sprinted down the path, and Glenn was watching. He runs down from the roof, soon getting down to the bottom, and watching you come closer

A couple minutes later, once you come closer, he opens the small fence, grabbing onto your arms, pulling you through the gate, and closing it before locking it, and running up the stairs, pulling you along.