American carriers have had support for RCS' next-generation text messaging for a while, but it typically hasn't been very useful outside of your home network. Unless you're with Sprint, you haven't had that universal support needed to chat between networks. Thankfully, that support is widening ever so slightly. T-Mobile is rolling out a software update that brings RCS Universal Profile to "select" Android devices. In theory, you can participate in 100-person group chats (among other benefits) with people using Sprint and other Universal Profile-enabled providers.
The only problem: at present, the update is rolling out solely to Galaxy S7 and S7 Edge users. If you're using a newer phone, you're out of luck. T-Mobile has promised wider support over time, but this won't help much if you're a Galaxy S9 owner expecting to get the latest and greatest features. It's a start, at least, and it does hint at a future where RCS is commonplace.