That is a great setup and an extremely low barrier of entry (which is cool). There are some webbased pieces of software, but they lack features making them tough to use as a group. For decent group rowing you need some infrastructure, both for organising events (what time to meet, etc..) and to do stuff together.
With EXR we tried to use the Discord channel for voice communication. Sound can be filtered and it can be heard well. The issue is that most people aim to stay in Zone 2, but after 30 minutes, they are huffing and puffing anyways.
Meeting afterwards typically is done on Strava or Discord, where people can drop some lines (speech isn't too popular somehow).
You need to get some metrics somehow. EXR runs on any Apple, Android or Windows device, and the Windows version can use the printer cable to a PM3 and PM4. And off-brand machines can connect (causing their own kind of issues, but hey, they are out there, paddling along). Most webbased free apps support the PM5 and FTMS, but don't cut it in terms of community. EXR basically has taken over the baton from RowPro here, many members used to row at RowPro.
EXR allows people to meet. This is how the larger organised group rows all started: people decided to meet at a given starting point at a given time. Our regular Saturday row was initially organied by Charlie Simpson (a former RowPro'er) and is now a group of 180 rowers of diferent pace, age and country of origin. Now EXR has taken over planning for the larger group rows.
Some smaller groups still meet this way: some start at 6:00AM CET to row together before breakfast. Somehow that group isn't too popular, but all five of them are there, every tuesday and thursday. Several of us have a group row at 17:00 CET. Discord allows us to publish events like that, polling interest for these events among everybody. They are planning to facilitate this more, but that is all in the future of monthly feature releases how easy it is to reach the entire EXR population.
But I also like to justrow and see who drops in. Last sunday I did a 21K, and 6K in a friend just dropped in. He planned for a 10K, liked the pace and stayed until finishing at his 15K. All is in community spirit and social contacts.