Signing in
Administrator and Staff sign-ins, and how sessions work.
One sign-in page serves two kinds of account, and the first thing it asks is which one you are.
Who can do this: everyone with an account.

Fill in Email address and Password, then press Sign in.
The page shows one message for a wrong email or a wrong password — it deliberately does not say which. For every message and its fix, see Troubleshooting sign-in.
Passwords & security · Invited staff: first sign-in · Troubleshooting sign-in