Select Organisation and Room to Enter
If you are a member of multiple organisations and rooms, you first need to go to the correct organisation page before you can enter the rooms in that organisation.
Select Organisation
The circled organisation displayed on the leftmost of the screen shows the organisation whose page and rooms you are currently viewing. Ensure that you are currently in the correct organisation.
You can check the name of a different organisation by hovering your cursor over the organisation logo. If you are in the wrong organisation, click on the logo to toggle to the correct organisation.
Select Room

In the left panel, you can now see the list of all the rooms that you have been added to in this organisation. Click on the name of the room you wish to enter.
Click Enter Room to enter a collaboration session with others.