Once you have entered a particular trip navigation is via the symbols at the bottom of the page. From left to right there are 3 icons as follows.
My Itinerary - This is where you land first when you enter a trip. It shows your itinerary for the trip
Travellers - This shows all travellers taking the trip. Touching a traveller's tile allows you to see their itinerary if this feature has been enabled by the trip creator. You can also see your own itinerary from this screen.
Messaging - This screen allows you to message other travellers. All messages created here are private to the trip.