Every world starts with nothing shared. From then on, discoveries are shared as players make them. If you add DiscoverySync to a world that has already progressed, a player can share everything their character already knows by typing this in chat:
/ds_sharealliknow
...or ds_sharealliknow in the F5 console. It is not a cheat command, so devcommands is not needed.
DiscoverySync compares the current list of items discovered since you installed the mod and adds only the difference to the server list. Players online receive them within a few seconds, and players who join later receive them when they join. Depending on the volume, these may be batched or show as individual updates.
The command is safe to run more than once, and the player is told how many items were new.
Who can run this command?
By default, only admins can run it, so someone bringing in an advanced character can't share spoilers. Server admins can let every player run it by turning off Restrict Share Known Items Command to Admins, or turn the command off entirely with Share Known Items Command Enabled.