Hi,
sorry for my english i will try to explain how to do it
if you want to let other users send in behalf of another user, go to active directory, go to the user who has the mail address you want to "share",right click it and go to properties. go to exchange general options go to delivery options and in send in name of select the users of group of users you want they can use that account for sending.
if you want to let users send in behalf of a public folder go to exchange administrator, first group of storage, go to public folder store,mailboxes right click the public folder you want to "share",right click it, go to exchange general options go to delivery options and in send in name of select the users of group of users you want they can use that account for sending.
i hope this will help
good luck