Dropping down to sit on the ground, Beverly just offers him the bag that she'd stolen. It's not warm anymore, but it's still good and coated with butter and salt. In other words the best way to have popcorn. She doesn't watch him as he takes it or he doesn't take it, or if he sits next to her or he doesn't. Beverly Marsh is someone who understands that how a person responds isn't about them, so she's not going to judge him if he stands or if he doesn't have popcorn: the invitation is there for him either way.