I think the only current way inside the platform is to invite someone into a voice chat or invite them into your Home Room or to an app that features chat. There's no text messages or leaving-a-message if that's the sort of thing you were looking for..
Although if you have the beta update that adds Dash, it'd be very easy to coordinate through any third-party messenger without taking the Rift off.
It's hard being the voice of reason when you're surrounded by unreasonable people.