I agree that it is time to re-teach! Maybe even assign those talkers a place to sit in the room, away from each other, where they can be monitored. I don't even introduce Read to Someone until near the end of school when they can read and possibly be able to stay on task working with a friend in a quiet environment. Could you place a responsible student near the door to be the door opener for those coming back in from another class?
My kindergarten students are doing really well this year at working independently and staying on task. In the past though, I have had groups who were talkative, distracted easily and unfocused. After reviewing, reteaching and practicing, if my students still chose to be off task, they would have to miss end of the day "center time" or "free choice" time and practice reading to self or working on their writing since they decided to play during work time. After a few times of missing the fun, they learned that they had better stay focused during Daily 5! Don't give up!