a platform you can jump-thru from the bottom, but cant fall through, is this possible?

If you're doing SinglePlayer you won't need to create a new 'CollisionZone' for each player, but the CollisionZone will just be an invisible part that represents the collisionbounds for multiplayer so that all players get the same view, but have their own interaction with it, allowing multiple players to jump through it at once.

--Assumes each platform creates a new 'CollisionZone' for each player added to the game
local function setCanCollide(collisionZone, collisionZonePlayer)
    --If the player is touching it, leave it at it's current CanCollide
    for i,v in pairs(collisionZone:GetTouchingParts()) do
        if v:IsDescendantOf(collisionZonePlayer.Character) then
            return collisionZone.CanCollide
        end
    end
    --Otherwise, the player is either above or below, and set CanCollide appropriately
    return collisionZonePlayer.Character.WorldPivot.Position.Y > collisionZone.Position.Y
end