In the header, below the Cocoon's name, is a display of all the members of your Cocoon. People who are online at this moment (viewing the Cocoon right now) will appear on the left, with a small dot next to their photo. People who are offline (not viewing the Cocoon) will appear greyed out, to the right. 

When you tap on an offline person:

If you tap on someone who is offline, you will see their name, when they were last in Cocoon, their local time, and a menu with a few different options for contacting them. 

When someone else comes online:

When another person enters the Cocoon while you are there too, they are displayed as online, and a colored dot appears next to their photo along with the banner message below. 

Tapping on their photo will take you to a special ephemeral chat experience that is only available to other people who are in the Coocon at that exact moment. More on this can be found in the article below.