If you are already a member of a workspace but haven't opened it in your Leverice for some reason, you can easily open a workspace and enjoy structured communication with your team following these steps:
Click the App menu button (the green one) in the upper left corner above the channel tree;
Select "Open Workspace";
Select or enter the email associated with the Workspace you want to open;
Choose the Workspace from the list below the email section.
A new Workspace is open!