When you've encouraged people to exchange information and views during a meeting especially by forming subgroups, a few people have trouble "shutting down" once you've called the meeting to order and you want participants to focus their attention in another direction. Whisperers and side conversationalists may not be consciously disrupting a meeting; they may just be finishing conversations or adding ideas. Nevertheless, you need a relative amount of silence and cooperation from them. Establishing eye contact sends a subtle message. Standing near them is another step. Stopping the meeting and maintaining silence until they stop talking is a more pointed measure. Asking, "Would you like to share your idea with the whole group ?" works as well.
If they persist, they may have a point to make such as an addition to agenda items or a need to voice a perspective not yet aired. You can ask, "Shall we add what you're discussing to the agenda?" Then, you can make a judgment call as to whether to give them "air time" then or call another meeting later to address their concerns. If they are simply bored or need to be the center of attention, the steps already mentioned should silence them. The idea is to control these persons so that the meeting progress is not slowed. A number of small conversations going on will disrupt a meeting.