The first step as a group is to draw circles. Use string and paper clips to try to help draw circles, or use other methods (see a video here). Then chose a color for each circle and shade in each corresponding section. This makes it easy to identify each field clearly.
<-- Using this example, students can write the names of people they know that fit each circle. The categories (from the center outward):
1. family and/or people you trust with secrets
3. classmates, neighbors, teammates, etc
4. acquaintances, relatives, people you don't know well or do not trust
5. strangers, workers in stores, etc
Depending on student issues and developmental levels, you may need to adjust the language and details accordingly. I suggest that you have students write in pencil at this point.
Once students have filled in their circles with names, pose a series of scenarios. They must determine with whom it would be most appropriate to share.
A few examples (that can be adjusted for your group's needs):
You have just received your test back from your teacher and you did not do as well as you had hoped. With whom would your share your disappointment?
You just bought a new shirt (toy, video game, etc) and you really like it! Who can know this information?
You have been having a medical issue (like lice, an allergic rash, etc) and someone asks you how you are. Who should you talk to about the details?
You are excited about going to a game, show, event, etc. Who would like to hear all about the details of your part in it?
Allow students to talk about people they know and how the scenario could fit in their own lives. Also, suggest they move names to other circles if they find it necessary (i.e., a student may find that a person they had in a middle circle may need to move to a circle with more or less private information shared, based on how safe they feel with that particularly person). When they feel good about their circles, they can go over the names in marker.
I believe this activity to be useful. The act of visually placing names and discussing what is safe and appropriate to share can help students to reflect on their current behaviors without any uncomfortable confrontation. After the group, discussions with individuals about behaviors or boundary issues can refer to the circles as a visual reference, too.