username@domain.com	realloginname
user@domain.com		user@somewhere.else
userlist@domain.com	listname

These can work many ways, as you can see.
Make sure you also add in the alias in the
alias file for list and for local delivery.
EG> You have a user name user1 and he wants 
his email to go to user@domain.com, use this
file and do the user@domain.com user1  into
the virtualusertable and then do a user: user1.
This way both the system and the real world know
where the mail goes.   

You'll also have to do the command:

makemap hash /path/to/virtualusertable < /path/to/virblah 

Remember to use the name you used in the creation of your
sendmail.mc file and replace as needed.

Then run the "newaliases" command to rebuild you alias file.

While I'm here. The third list is just that a list. Put that line in
where list is the name of the list and userlist is the alias. Doesn't
have to be a user, after all this is the virtual user table. Using the
list simple put where it goes. eith local should look like this:
username
username1

You can also do this
username@domain.com    

As long as it's not local, thought I forget. Then add a line to
you alias file: 
listname:	:include:/path/to/list

Then rebuild aliases. As I run when I rebuild any of the files
I always end with the m4 sendmailmc > sendmail.cf  and then restart
sendmail.