Update: Turns out the app is out already. AT&T Enhanced Push-To-Talk
The company said that the service would support up to 1000 contacts and group calling with up to 250 people.
Push-to-talk was a popular feature offered by Nextel which is now owned by Sprint. Incidentally, there are a bunch of smartphone apps that offer a similar functionality but they require a data connection.
AT&T’s service is also likely to work through an app. It is not clear if it would work on cellular connections or would need data as well.