how to mount share as a different user?

i am sorry if this has been covered here before, my brief search didn't come up with anything and i think there's a really simple solution.

i have never been able to figure out how to mount a share using credentials other than the user who's logged into the computer already.

for example, my company has certain share points that only a few of us can access. if i am at a user's computer, and they've already mounted a share point that everyone can access, AND they saved their username/password in their keychain, i am not able to log into the the same server using MY credentials. it saves their username/pwd in the keychain.

in windows, if i wanted to access a share point that only i have privileges to from another user's computer who does not have those privileges, i can select "log in as different user" - but i have never seen anything like that in OS X.

the only work around i know of is to delete the user's keychain relating to AFP logins.

btw, all users have local accounts. we don't use OD.

thanks in advance.