To keep all your guests in your park, put a no entry sign at the entrance turned around so they cannot exit. After getting the required number of guests in your park, raise the land right before the exit/entrance to the park. Go to the entrance of your park and build a bridge. Then, make a hole under the bridge so the people will not get out. Get the desired number of guests into your park, then remove the piece of land at the start of your park entrance.