Sena units with " mesh tech"
Easiest fastest way to link all in a group.
We've been using these on bike trips for the last few years and they're pretty good. The increased range (compared to bluetooth) is nice for when you have a large group or get spread out for whatever reason. Aside from communication they also are good for playing music or directions (if you have a trail GPS onto your phone).
The only con of the mesh network compared to bluetooth is that the battery consumption is greatly increased, if you're going to be riding all day I'd definitely plan to take a powerbank to charge it during lunch or any extended stops. We generally do 8+ hours of riding in a day on our bike trips and I've not run out of battery yet with just charging it at lunch.
One question I have is if I get something different from what he has will the 2 units be able to communicate? Are they like an FRS walkie-talkie where it doesn't matter what brand you get they all talk to each other?
Bluetooth connections between different brands are possible, but can be a pain to setup and is more limited in the number of users (generally you can only connect to 1 other person). Mesh is much easier to setup and can support more users. It is possible with some of the newer headset models to get multiple brands on the same mesh network, but that can also be difficult to setup.
We're mostly running various Sena models, I have a 50R and a couple of guys have the 50S. A few guys have Cardos and usually the first night we're at a location is spent messing with the helmets trying to get it setup. Sena and Cardo don't intuitively connect to the same mesh but if you connect a Sena and a Cardo via bluetooth, then the Cardo user can get brought into the Sena mesh with some restriction - though the Cardo user can hear everything transmitted over the Sena mesh, I think they can only talk to the specific Sena user to whom they're connected via bluetooth. I'm not sure if that has been addressed in newer models, as ours are all over 3 years old at this point.
If nobody in the group already has a headset, then it'll definitely be easier if you all agree to get the same brand (mixing models within a brand shouldn't be an issue).