To collaborate with other users on a Project, you must first share it with them to add them as members. You can display the list of the members of a Project by clicking the members icon in the top-right corner. 

There are several ways to share a Project:

• From the Projects tab: Swipe left on the title, click the members icon and select your desired sharing option
• From within a Project: Click the members icon in the top-right corner and select your desired sharing option
• From the list of members: Click the icon in the top-right corner of the list of members for sharing 

The recipient will simply have to click on the sharing link. As soon as they join the Project, their username will appear in the list of members.