If you have been invited into a Project by someone then you will receive an email-invitation outlining: who has invited you; what the project is; and what your role/discipline on the project is. You have to log-in to the website to accept or decline this invitation as follows:
After you have clicked "accept" you should see the project appear under "My Projects" under the sub-heading "I am a member". You can click on this project to browse the project and see the documents you are responsible for under "My Documents".