Grin could be added to the Mixin protocol by following the examples of other domains here: mixin/domains at master · MixinNetwork/mixin · GitHub
Once it is added to the domains, I’m not totally sure what needs to happen after that, but if a crypto currency is added to the protocol, it can then be used in the app.
The benefit of grin being in the app is that it provides a full messaging network where people would be able to pass slatepacks in a pretty way.
This thread is to conduct research together on the possibility of this. It could be a good example of a better messaging UX for grin that doesn’t require something being added to the core