Email domain invite
If your institution has Email domain invites enabled and you are signed up with your institution's email address you will see an invite on your course overview page. (see below)

When you click on Join institution your account is now a part of the institution. 

If you clicked on Dismiss, and don't show again you will have to contact one of your institution admins to get an invitation by email.

Email invitation
Your institution admins are able to send invitation through email. When you receive the email you will need to accept the invite. Once you've accepted you're a part of the institution!