One Idea would be that everyone who you happened to see on a Server gets stored in a List, so that you can somewhat keep track of if you saw that person before or not. Also makes it easier to “friend” people if they are sorted by how often you met them.
There will probably be a range requirement for someone to be considered as “have met”, or just pinging someone in Chat (or having been pinged) could count as “have met” too. Everyone else would be considered as “was on same server, but is unknown”.
Though if there is a Range requirement, that Range has to be VERY low, otherwise you can exploit that as a Radar.