OSF accounts are free to everyone, but you need to login through JMU to affiliate your work with the JMU Institution page.
Use the Sign Up button on the OSF home page.
Then choose 'Login through your institution'
Then select JMU from the pull down menu.
You will then be prompted for your JMU eID and password using the usual JMU branded, Shibboleth authentication screen.
Use the Sign In button on the home page.
Then use the "Login through Your Institution" link.
Select JMU from the Institution pull down menu as above.
Enter your JMU eID and password on the JMU branded, Shibboleth authentication page.
You will then see your Dashboard.