Assuming you know that others should be there with you, here are a few tips to fix the issue.
First, make sure you're using a compatible device and browser. This is the most common cause of showing up "alone" in a space where you know there should be others.
If you see the background but don't hear or see other avatars, restart your computer or connect from another device or network (such as your phone over 4G).
If you are able to connect from one device but not from another compatible device and browser, please report the issue so we can try and figure out what happened.