You can create new accounts from the Edit Users page. Start by clicking the "Add" button in the upper right, and then selecting "User". If you want to grant access to many people at once, please see this article.
You'll see a few fields to fill in. 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.
If the user's name is listed in the "Link to Staff Member" dropdown (3), be sure to choose it. It's okay if it's not listed though. You can continue on in the user creation process.
Choose which group(s) this user should be a part of (4). Group memberships determine which permissions this user will have – i.e. what they can and cannot see in Schoolrunner.
Lastly, choose the school(s) to which this user should have access (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! S/he will receive a welcome email with a link to log in.