It's easy to create new user accounts individually. If you need to create a large number of users, please see this article on bulk creating users.
Start on the Configure page, where you'll see an "Edit Users" link under the "People" section. (Note: You may need the correct permission if you don't see this link.)
Click the "Add User" button to get to the "Add User" screen.
You'll see a few fields to fill in. Make sure to enter the user's first and last names (1) and email address (2). The email address you enter in this field will become the username for this account, so ensure it's spelled correctly and doesn't have any extra spaces before or after it. Users are generally tied to work/school-issued email addresses, which is recommended.
A user account must be linked to a staff member record (3) in order for that user to be able to log behaviors, communications, incidents, etc. Linking a staff member record to a user account isn't strictly necessary, but is also recommended. Read more about the distinction between staff members and users.
Lastly, choose the school(s) to which this user should have access to (5). Even if you are a single-school client, you must choose the school from this dropdown or else the user won't be able to log in to Schoolrunner at all.
Once you click the "Save" button, the new user has access to Schoolrunner!
In order to log in, s/he should be instructed to visit your Schoolrunner login page, click the "Forgot your password?" link, and enter in the email address you just assigned to his/her new user account into the email field, and hit the "Reset Password" button. Schoolrunner will then send a login link to that email address.