Simple website that will dipslay you all players connected to selected sever. It is available here: https://igorovh.github.io/fivem-player-list/
It is using FiveM endpoint, which is used in their UI:
https://servers-frontend.fivem.net/api/servers/single/serverId
Visit FiveM Server Browser and click on chosen server.
After that look into the URL and copy last fragment, which is Server ID. Example:
https://servers.fivem.net/servers/detail/vp4rxq
-> vp4rxq
Find server in FiveM client and click on it.
In the right side you will have "join URL", in which last fragment is Server ID. Example:
cfx.re/join/vp4rxq
-> vp4rxq