To invite fellow travellers to your trip, you first need to click on the trip you want to invite others to. On the left side you will find the yellow navigation bar. By clicking on the settings icon, the following dialog will open.
In the attendees section you will find the Invite friends field. Here you have two options to invite other travelers:
1. You can either enter the email address or username of the other traveler. The person will receive an invitation to the trip and can join the trip by accepting the invitation.
2. Or the person enters the displayed PIN code and can join the trip using this method.