Cerberus has been one of the most popular device security apps over the years, and the developers are now looking to protect more than your phone. Persona is a "personal safety" app that can instantly share your location in the event of an emergency. This service is included with Cerberus, so subscribers are already set as soon as the app goes live.
Persona is out in beta today, with the opt-in available at this link, and the app will be here. Persona uses your Cerberus login information, but new users can try it for one week free, after which you'll need a Cerberus subscription (it's just a few bucks per year). Persona can relay your live location to your chosen contacts via SMS, email, Facebook, or Twitter. You can also set the amount of time the link works.
Cerberus is supposed to be quicker and easier than other location sharing options. Thus, you don't need to open the app to start sharing your location. There are widgets, shortcuts, and even an Android Wear client. Your contacts can be assembled into groups as well, allowing you to share with different people under different circumstances. The testing link appears to still be rolling out, so give it a little while.