Great idea behind this bot function. While it's fun for doing prank to friends, this function also can be used to do bad thing.
Maybe if you can add security function like if the sender going to send sms to more than 20 peoples, reject the sms.
Or something like counter to count the number of recipient, so if already used to send to many receiver, will trigger the failsafe alarm.
the feature on this bot just able to send 1 message a time so sending to 20 people will need you to send it 20x
maybe the word counter in the bot is good. so the counter needed before the people send the message and it will show how many char/symbol already in the message