Or Signal.
But even tptacek is on record here on HN now admitting that even unencrypted sms or iMessages are fine as long as you don't need bulletproof crypto.
Set up a Google group, Reddit sub or other standard platform for your building.
This will depend a lot on how well you know your neighbors, what their default digital platforms are and how compatible everyone is in that regard.
Notes posted in the lobby. Going door to door to talk to people...