Here's a quick tutorial on how you can invite a staff member online.
Login to your account in the Online Dashboard, and go to Settings
Click Staff
Click Invite Staff
Enter the details of the staff member, then click Next
Click Finish
The staff member will receive an invitation through SMS and/or an email message with their login details.
Review staff Security Roles
Check out this article on Security Roles, which explains the levels of access which come with each of ServiceM8's default Security Roles, and how this can be managed.