Sign in with the email address and password you were given. There is no self-service sign-up yet; an administrator creates accounts.
Locked out? After five failed attempts in fifteen minutes, sign-in for that email address (and from that network address) is paused. The message says how long to wait. Waiting is the only fix; the pause clears itself.
Signed out unexpectedly? Sessions end after a period without activity (set on the Settings page, eight hours by default). Signing in again returns you to the page you were on.