The best way is for you to create them accounts and tell them what their usernames and passwords are. If you only have a few you can make these manually in site admin>users>accounts but if you have a lot you can do it using a csv file which is a plain text file (you can do in Excel and save as csv) with the fields
username, password,firstname, lastname, email
(you can use a fake email if they don't have a real one) Then you upload this to users>accounts>upload users.