How do i contact a fellow passenger? like a co-rider on an UberPool or LyftLine?
The Guru Take
This is not permitted. Privacy concerns.
There is no way for you to contact your fellow passengers unless you exchanged information while on the ride.
Friendly conversations are great and even encouraged while on pooled rides, but unwanted contacts are not appropriate and go against community guidelines of both rideshare companies.
Side note: Some peple claim this as one of the biggest setbacks to shared rides, where the security and privacy can be a concern. Fellow riders may be able to see where you work or where you live. Be safe!
If you are really fond of someone and had a good connection, perhaps you can search for "Missed Connection" sites or a section on Craigsist, and you can post your information there?