The Organisation Bar
Here you can see the logo and the name of your organisation, your username with which you are logged in and the settings icon where you can edit your settings.
The Menu Bar
Here you open the conversation settings and can access employee/coworker lists as well as your group lists.
At the top of the list you can see your pinned (starred) conversations, if you have any. Below, you'll find all other conversations, sorted by most recent message first.
Pinning (Staring) a Conversation
This is as easy as pie. Just click on the star right next to the conversation name of the opened conversation. Another click removes it again.
Here you can search the conversation and open the enter key with just one click. You can also open conversations that you haven't yet joined.