Click on + Start New to create a new channel. This will present you with a list of your Plays to select from, or you can create a blank channel. Once created, other users can join your channel.
When you have selected to start a play or blank channel, you will have the option to click on Advanced settings and alert all invited users that this channel was created. This will send an email notification to all personnel invited to that channel. You can set the default for this in Plays.
Click on the orange + button in the bottom right hand corner of your phone to create a new channel.