Install FAM, start FAM and it will detect a account, name the account and close FAM
Create a new temp windows login account (Control Panel > User Accounts)
Switch to new windows temp login account
Start freelancer, select multiplayer and it will give you a new account key, click OK and quit freelancer when you get to the server list screen
Start FAM and it will ask you if you want to add the new account, click yes
Log out of the temp windows login account and log into your normal windows account
Delete the temporary windows account
Open FAM and you should now have two accounts
Rinse and repeat for more accounts as neeeded, just dont use the same name for a temp windows login account
Install...done. You can now change accounts by double clicking on them.
You can manually add an account using the drop down "Accounts" menu option or follow kindred intructions for a new one.
Don't know if theres an easier way, may be quite complex for a new user to mess with windows acconts.