Off-line communication

I understand private messaging from within the App whilst in a ride, but does the follower have to be in a ride/the same ride to receive a private message or can they just get it through the app the next time they open it?
I want to be able to contact a follower and the only communication channel I have is Zwift. I understand that I can invite them to meet by inviting them to a time and place to ride but I want to be able to say for example “What days and times do you have this week for a ride together?”. Can you make that possible anywhere in Zwift?