Adding friends enables you to check their calendar online and find out when they´re free right away. You also have an opportunity to add badges into your friends´ calendar, inviting them to an event or activity.
When they accept your invitation, a chat between you two opens up and you can discuss the details. You can open the conversation by clicking on the badge added and add other people to the conversation as well. Other people will only get the invitation if they´re free at that time.
In case you´re not sharing an event with anyone, the chat window will provide you space for your notes. Chat you share with your friends is, of course, accessible for people involved only.