There are number of ways to start adding your volunteers to your organization site on Track it Forward.
Import contacts via a spreadsheet or add one at a time in the Organizer Dashboard.
This method is done by the Organizer or other admin in the Organizer Dashboard > Volunteers
Basic volunteer information is added here like name and email. You can set a password for the volunteers and then send an invitation to them by checking the box for Send Invitiation. The invite gives them instructions on how to log in and prompts them to If you would like to add to the invitation it has an email template that you can find in Settings > More Settings > Email Template. Just be sure when you add to this you keep the code that represents the link to your site. You can also invite specific people in your volunteer list by choosing "Send Invite" as your action when selecting names in the list of volunteers.
If you as the Organizer prefer to keep everyone organized by a distinct ID number this is the method you would use. The information fields here can be configured in the Settings > Volunteer profile
Have your volunteers search for your organization on the Track it Forward home page.
This one requires that your site be made publicly searchable. http://help.trackitforward.com/knowledge_base/topics/make-your-site-publicly-searchable-on-the-find-a-site-page
When they find your site they can register themselves as a new user with your site and if you have it set to allow new users without approval then they can start logging hours immediately.