I am assuming that you don't have user accounts set up already, right?
If you've never done this before, start here with The Linux Documentation Project
You might want to install "pwgen" to generate initial secure passwords for your users.
Once you have created user accounts, each user can log in with the credentials you provide (i.e. through SSH). Be careful in the way you notify users about their new acccounts!
Do NOT simply email them, since the clear-text can be intercepted.