For years, AT&T has been trying to push its SMS successor, Advanced Messages. The problem is that the standard only supports a handful of flagships and isn't even compatible with the more universal RCS standard, let alone other carriers' insular solutions. Still, AT&T is working to get Advanced Messaging out to more devices and has just pushed out one such update to the Samsung Galaxy Note8 as well as the S8 and S8+.
All three models have received the capability via an Android 9.0 Pie system update from October 15, which also raised the security patch level to the one from August. With the help of Advanced Messaging, you can send larger files and create bigger groups, and you'll see delivery and read receipts and typing indicators. You shouldn't need to download an additional app to start using it, the functionality should be available in your standard messaging application.
Samsung Galaxy Note8 (N950U), S8/S8+ (G950U) / (G955U)
What's new: Advanced Messaging v2
Release date: October 15, 2019
Android version: 9.0
Security patch level (SPL): August 1, 2019
Baseband version: N950USQU6DSH3 (Note8), G950USQU6DSH8 (S8), G955USQU6DSH8 (S8+)
Build number: PPR1.180610.011.N950USQU6DSH3 (Note8), PPR1.180610.011.G950USQU6DSH8 (S8), PPR1.180610.011.G955USQU6DSH8 (S8+)
File size: 600MB (Note8), 418MB (S8), 415MB (S8+)
To see if the system update is already available to you, head to your device's Settings and scroll down to Software update. Tap that, and then look for the Check for updates button. Since OS upgrades roll out in stages, it could take a while until it's visible to you, though.