Be sure students are able to work independently first - Working with a small group while the rest of the students work independently is one of the most difficult - if not the most difficult - activities to manage. Therefore, before you attempt this activity, make sure the students have demonstrated that they can work independently at their seats.
Make learning how to work independently your first priority - At the beginning of the year when you have the students working at their seats while you are with a small group, your job is to make sure the students learn how to work independently at their seats. Make your top priority monitoring the students working independently rather than teaching the students in your small group. Until the students at their seats learn to stay engaged and quiet on their own, no meaningful learning will take place.
Working In pairs - Your next priority after independent work is making sure students can work well in pairs. There may be times with you have them working in pairs while you are working with a group. You have to be assured that they are doing a learning activity with their partner and not just messing around.
Determine how students can get help - Because other students cannot ask for help when you are with a group, talk with the class beforehand about what they can do other than wait for you. With some prompting, students will come up with actions such as asking a friend for help, leaving the part they do not understand unfinished and moving on to another section, or trying the problem the way they think would be correct.