A free app for Android, by Crazy Broadcast.
Multi SMS Sender
MSS
is an easy-to-use and powerful messaging app that can send messages to multiple people at the same time. It allows you to create groups, add members to those groups, manage the messages you send to them and their status, and create custom messages.
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You can manage all the contacts in your phonebook or create new ones, adding their phone number or email address. You can search through the list of contacts to find the ones you want to send a message to.
It is a very easy-to-use app that does not require any special knowledge to use. You can use it without having to install any additional plugins.