Board the same physical train as your friends.
Pick the train you're on and TrainSync mints a code like 6-MAPLE-S — derived from the train itself (line, direction, and the minute it left its terminal), so every phone computes the same code for the same train. Share it; anyone who enters it sees this train's live arrival time at every remaining stop, straight from the MTA's real-time feed. No accounts, no server — the code is the train.
I watched this video a while ago, and thought it was cool how they were able to time it so that they could get onto the same train at different stations. I’ve done similar with my friends before who live off of different stations from me, and it’s always a bit of a hassle to figure out which train they should get on, that’s the same as the one you’re getting on.
Times are the MTA's live estimates and shift with holds and reroutes. Trains appear in the feed only ~30 minutes before leaving their terminal, so you can coordinate the next train, not tomorrow's. Shuttles aren't included.