1. someuser@somehost is a repersentation of a
user's id at (@) some host BUT it normally
represents an email address and does not
necessarily mean the user has anything but
an email account on that server. In other
words, it has nothing to do with telnet access.

2. you would use the "su" command to change to
another user assuming you have that user's
password. Check the man page su(1).

I hope this answers your questions. I'm sure
it creates more