Think of it this way: If George Clooney shows up for 20 seconds in a two hour independent film, the film becomes about that 20 seconds. Now you have an hour and a half of wondering if someone else from Oceans 11 might pop up. It would be unfair to the new characters if they bump into Glenn and Maggie while they are out on a supply run. At that point everyone would be focusing on the possible crossovers, future encounters, a movie version featuring both crews (a la Star Trek Generations) rather than the story at hand. Rick and company have plenty to contend with in their own little corner of paradise. Leave them to it.