If your friends/colleagues are also using Cubase, they can simply send you the project folder (including the project file and supporting audio/MIDI files). If they are on another DAW, best to have them export each individual track as a separate .wav file, which you can then import on your side and add your parts. One drawback is that you might not have access to the same plugins that they are using, but they could always print the effects to a track before they send it to you.
A good way to do this is via an online file sharing site, as project files are typically too large to effectively email. I use Box for this when collaborating on others' project files. Dropbox, Google Drive, etc. also work.
