A social update is due soon to allow this to be easyer.
Only way at the moment i know of is via the phone app, If you select a rider from the peek list (riders near you) you can see their profile and follow them and give ride on. I think you can also see anyone who has given a ride on during a previous ride and follow them that way too.
That was just my question today! How do I find my friends/Strava follwers in Zwift? Would be great if I could add them even if they’re not online - finding them when 1000 riders are online would also not be that easy.