Do an action for all players online, such as add a temporary permission or prefix.

Basically /lp user (online) [any action here]<-- To select all online players.

Such as /lp user (online) permission settemp (permission)

Out of scope for LuckPerms in my opinion.

There's probably a 3rd party plugin that will allow you to run a command for all online players.

Something along the lines of /runforall lp user <player> permission settemp something true 1d