There may be two or more members in a project, the project owner can invite member into the project.
Click on the invite button in the top left corner in project page, input the invitees' email and click on the invite button.
After the invitation, the invitee will get an emial. And he/she can just click on the agree button in that email to get into the project.
Besides, you can also copy the generated specific link and share it with the invitee. He/She can get into your project by visiting the link.