How to make a part invisblelize a player??

script.Parent.Touched:Connect(function(plr)
    if plr.Parent:FindFirstChild("Humanoid") then
        for i, v in pairs(plr.Parent:GetChildren()) do
            if v:IsA("BasePart") then
                v.Transparency = 1
            end
            if v:IsA("Accessory") then
                v.Handle.Transparency = 1
                for index, value in pairs(v.Handle:GetDescendants()) do
                    if value:IsA("ParticleEmitter") then
                        value.Transparency = NumberSequence.new(1)
                    end
                end
            end
        end
        plr.Parent.Head.face.Transparency = 1
    end
end)
script.Parent.TouchEnded:Connect(function(plr)
    if plr.Parent:FindFirstChild("Humanoid") then
        for i, v in pairs(plr.Parent:GetChildren()) do
            if v:IsA("BasePart") then
                v.Transparency = 0
            end
            if v:IsA("Accessory") then
                v.Handle.Transparency = 0
                for index, value in pairs(v.Handle:GetDescendants()) do
                    if value:IsA("ParticleEmitter") then
                        value.Transparency = NumberSequence.new(0)
                    end
                end
            end
        end
        plr.Parent.Head.face.Transparency = 0
        plr.Parent.HumanoidRootPart.Transparency = 1
    end
end)

Insert this into a part.

This code is the same as @bebokhouja2's answer but with explanations and few improvisations.

script.Parent.Touched:Connect(function(hit) -- when the player touches the part
    local character = (game:GetService("Players"):GetPlayerFromCharacter(hit.Parent) ~= nil) and hit.Parent -- checks if hit is the player's limb
    if character:FindFirstChildOfClass("Humanoid") then -- checks if the character has the "Humanoid" instance; detects if it's a real alive roblox character
        for i, v in pairs(character:GetDescendant()) do -- gets all of its descendants (the children of children of children of ... children of a part)
            if v:IsA("BasePart") then -- checks if it's a part/limb
                v.Transparency = 1 -- sets the limb invisible
            elseif v:IsA("Accessory") then -- if it's an accessory (e.g., Hat, Backpack, etc.)
                v.Handle.Transparency = 1 -- sets the accessory invisible
                for index, value in pairs(v.Handle:GetDescendants()) do -- gets its descendants
                    if value:IsA("ParticleEmitter") then -- if it's a particle
                        value.Transparency = NumberSequence.new(1) -- sets the particle to invisible
                    end
                end
            end
        end
        character.Head.face.Transparency = 1 -- sets the face invisible
    end
end)
script.Parent.TouchEnded:Connect(function(hit) -- when the player leaves the part
    local character = (game:GetService("Players"):GetPlayerFromCharacter(hit.Parent) ~= nil) and hit.Parent -- checks if hit is the player's limb
    if character:FindFirstChildOfClass("Humanoid") then -- checks if the character has the "Humanoid" instance; detects if it's a real alive roblox character
        for i, v in pairs(character:GetDescendants()) do -- gets all of its descendants (the children of children of children of ... children of a part)
            if v:IsA("BasePart") then -- checks if it's a part/limb
                v.Transparency = 0 -- sets the limb visible again
            elseif v:IsA("Accessory") then -- if it's an accessory (e.g., Hat, Backpack, etc.)
                v.Handle.Transparency = 0 -- sets the accessory visible again
                for index, value in pairs(v.Handle:GetDescendants()) do -- gets its descendants
                    if value:IsA("ParticleEmitter") then -- if it's a particle
                        value.Transparency = NumberSequence.new(0) -- sets the particle to visible again
                    end
                end
            end
        end
        character.Head.face.Transparency = 0 -- sets the face invisible
        character.HumanoidRootPart.Transparency = 1 -- sets the rootPart to invisible otherwise the character would look glitchy and weird
    end
end)