Why should I register my phone number?
If you register your phone number, your friend can know whether you use Awesome Calendar or not.
In contrast, if your friend register his phone number, you can know that he use Awesome Calendar.
When Awesome Calendar is launching, it search your friend list using Awesome Calendar. So you can see highlighted friend list with orange color like attachment.
Then you can send your event, todo and notes(ebbed photo) to your friend using Awesome Calendar via DIRECTLY, NOT email or SMS.
Like below image, friends using AwsomeCalendar will be displayed by orange color in contact list.
You can learn more about sharing, see below link.