To join an existing trip, click on + NEW TRIP in the navigation bar at the top, while you are in your trip list and choose Join trip.

Afterwards, enter your 6-digit join-code and click on JOIN TRIP NOW.

Alternatively, if your trip list is still empty, you can click on JOIN TRIP on the welcome screen:

To find out how to join a trip using the Lambus iOS / Android app, please read this article.