Create a text file named "whitelist.txt" in the vrchat installation folder (where vrchat.exe is).
There, add VRChat user ids, one per line, to the file.
Everyone not on the list will not exist for you ingame.
Controls
F8 to disable the whitelist (you'll need to leave and join the instance for the changes to apply).
F9 to save a whitelist snapshot of the users in the current instance to the vrchat install folder. This snapshot will be named vrcwhitelist_snapshot_<time of snapshot>.txt.