Users Unable to Log In to Private Blog

I’ve set up a private blog about my son and have invited a few family members to create WordPress accounts so that they may view it. I’ve also created a test account for my own troubleshooting purposes. I successfully created and verified the account and have logged in. When I go to the blog URL, I’m prompted to log in and I do so using the verified username/password. My log in is unsuccessful, and I’m just sent right back to the log in screen.

Any help is greatly appreciated! I’m hoping to make this a foolproof process, especially considering that I’ll be inviting a lot of older, less computer-savvy family members.