The messages worth sending to a whole building
Anything that changes a tenant's day and applies to everyone. The water is off between nine and one. The lift is out until Thursday. Contractors will be on the roof from Monday. The car park is closed for resurfacing.
These are the messages that generate phone calls when they are not sent, and the calls always come to you. One text to the building costs five dollars and removes most of them.
A working example: OAK STREET APARTMENTS: water off Tuesday 9am to 1pm for pipe repair. Please fill what you need Monday night.
Why a text is not a legal notice
This is the part that matters most. Almost every state sets specific rules for how certain notices must be served, including notice of entry, rent increases, lease termination and eviction. Those rules generally require written notice delivered in a prescribed way, and a text message usually does not satisfy them.
Treat text as the courtesy layer that sits on top. Serve the formal notice however your state requires, then text tenants to tell them it is coming, which reduces the number who claim they never saw it.
If you are unsure what your state requires, that is a question for a local attorney or your state's landlord-tenant guidance, not something to work out from a blog post.
Emergencies and things happening today
Messages go at 12pm Eastern on the chosen date, which suits planned work well. It does not suit a burst pipe at eleven at night, and it should not be relied on for anything genuinely urgent.
For a real emergency, phone the affected tenants. A broadcast is for the twenty people who need to know the water will be off tomorrow, not the one person whose ceiling is currently coming down.
The useful pattern for weather is the day before. Storm forecast Thursday, please move cars off the lower lot by 6pm is a message that saves everyone a bad morning.
Building the tenant list
A QR code on the noticeboard in the lobby and a line in the welcome pack when someone moves in. Tenants scan it and add their own number, which means you are not texting numbers pulled from an application form.
Say plainly what it is for. Building notices and maintenance only is a promise tenants will accept, and it also sets the expectation that you will not be using it to chase anything.
Tenants who move out can remove themselves by replying STOP, though it is worth refreshing the list periodically anyway. Rent reminders are a separate matter and are better kept on a separate list.
Multiple buildings and what this cannot do
One list means one message to everybody on it. If you have three buildings and the water is only off in one, you need three separate lists rather than one list with groups inside it.
There is also no record-keeping built for disputes. You will know a message was sent, but this is not a system designed to prove service of notice, and it should not be used as one.
For a portfolio large enough that this gets awkward, property management software with tenant messaging built in is the better purchase. For a landlord with one building and twenty units, the way HOAs handle community-wide alerts is a close comparison.