Can you check if a player is in a table from another script?

I think you are going to need to post more context, but here is my answer from I can obtain.

Assuming both scripts are server sided:

local event = game.ReplicatedStorage.BindableEvent  -- some event you have

event.Event:Connect(function(players)
    for i, player in pairs(players)  do
        local char = player.Character or player.CharacterAdded:Wait()

        char.Humanoid.WalkSpeed += 10 -- increases speed
    end
end)


-- in the main script

local event = game.ReplicatedStorage.BindableEvent

local players = {} -- your table

event:Fire(players) -- this is how you fire the server, call this whenever you need to increase the speed

The most efficient option is to use ModuleScripts, as you can then access tables directly (if you put the table in what is returned by the Module), example:

Module Script:

local module = {}
local someTable = {}
module.SomeTable = someTable
-- can use someTable here
return module

Script:

local Module = require(path.to.module.script.here)
-- You can now use Module.SomeTable here