If you already have a JAMA Network account, you can use your current credentials to access the site. Go to https://jamanetwork.com/ and click on the Sign In dropdown menu located in the upper righthand corner of the home page and follow the subsequent prompts.
If you do not have a JAMA Network account and need to create one, click on the same Sign In menu, select Create an Account and follow the subsequent prompts. It is free to create a JAMA Network online account.
Once you have signed into one journal site, there is no need to sign in to the other JAMA Network sites. Your login will carry from site to site.