Do any friends know any convenient ways to send short messages in groups?

You don't need to be too responsible If you send a large amount, you can use onesmser SMS platform, log in on your computer or your mobile phone. This restriction is much less, and all the problems you mentioned above can be solved. And the price is much cheaper than your mobile phone, about 60 cents each. If the amount is large, it can save you a lot of money.