If you have several logins for a site that you would like to save in LastPass, simply log in to each account as you normally would. LastPass will recognize when a new username/password combination has been entered and will prompt you to save this new account.
The next time you visit the site, LastPass will show the number of logins saved for this site at the toolbar icon and at the field icon. Clicking on the field icon will provide a dropdown list of sites:
LastPass also autofills the login form with its best guess for which account to use. If you would like to log in using a different username/password, click on the AutoFill or AutoLogin button in the dropdown menu to get a list of all of your saved accounts for the site. This list contains the name that you gave this LastPass site and the username. LastPass will autofill or autologin when you choose an entry from either dropdown list.
You may find that LastPass is designed to offer logins from other parts of the site on a page, even if only the domain is the same. You’ll also find that the top-most listed sites are the ones that best match the page you’re currently on.