The two halves of doing this properly
Consent comes first. Someone has to have agreed to receive messages from you before the first one arrives, and a phone number given for another purpose is not agreement.
The way out comes second. Every message needs to tell the recipient how to stop, and stopping has to actually work without them having to chase anyone.
Most compliance problems come from getting the first part wrong rather than the second. Adding STOP to a message sent to people who never agreed to hear from you does not fix anything.
What STOP and HELP have to do
STOP is the universal exit. When someone replies with it, they are removed and no further messages reach them. This happens at the carrier level, so it works whether or not the sender does anything.
Carriers also recognise variations, and a compliant setup honours the common ones rather than only the exact word.
HELP is the other required keyword. Someone replying HELP should get a response identifying who is messaging them and how to get support, which matters because recipients sometimes forget what they signed up for.
Collecting consent that actually holds up
The cleanest form of consent is the person taking the action themselves. A sign-up link or a QR code they scan means they chose to join, and there is no argument about it later.
What causes trouble is uploading numbers gathered for another reason: booking forms, membership directories, order records, an old spreadsheet. Those numbers were given for a specific purpose and reusing them for messaging is where organisations get into difficulty.
It is also worth telling people what they are signing up for at the moment they join. How small businesses build a customer list covers the practical side of this.
Why doing this from a personal phone is harder than it looks
If you text a hundred people from your own phone, you are the compliance mechanism. You have to track who asked to stop, remember not to include them next time, and answer anyone asking who you are.
One mistake, such as a name left on a list after they asked to be removed, is exactly the kind of error that causes problems.
A service removes that burden by handling it automatically. That is a real reason to use one beyond convenience, particularly for anything organisational or commercial. Sending to a large list covers why phones also fail on delivery at scale.
Where the rules apply and where they do not
The strictest requirements apply to messages sent for a commercial or organisational purpose. Texting your own friends about a birthday party is not what these rules were written for.
The line gets blurry in the middle, such as an HOA, a volunteer group or a club. The sensible approach is to treat anything going to more than a handful of people as though the rules apply, because the cost of doing so is nothing.
None of this is legal advice, and the details vary by situation. Anyone with a specific question about their own compliance position should ask an attorney rather than rely on general guidance.