To become the shared user, simply type
become pseudouser(where pseudouser is the name of the shared user). When you're done being that user, logout will return you to your own identity.
Users sharing the account can either log into it to read email sent to the shared account or use ~pseudouser/.forward to redistribute it. If you would like two other people to receive email sent to pseudouser, ~pseudouser/.forward should look something like this:
user1, user2If you want to be able to reply from the shared user, it will have to receive copies of the email too. ~pseudouser/.forward should then look something like this:
\pseudouser, user1, user2