A Bubble Pin is one of three ways to join a Bubble. You do not need a pin to join a bubble if you can be invited to the Bubble by the owner or a participant (if privacy settings allow.)
The three ways to join a Bubble are:
1) The owner of a Bubble can add you to the Bubble. You will receive an email notification when you have been invited.
2) If the owner of the Bubble has selected to allow participants to invite others (under the Bubble's Privacy Settings) a Bubble participant can also invite you to that Bubble.
3) If you have a Bubble's Pin, you can join a Bubble with that Pin without having been invited to the Bubble.