Why is it saying that I'm staff instead of guest?

Problem

Why is it saying that I'm staff instead of guest?

Whats wrong

First you're not even comparing if the player's UserId and the userid when iterating are equal. Second you're overwriting your original reference to userid. In this case you're checking if userid has truthiness. Strings have truthiness and so the conditional will always be true

Example

if "abc" then
    print"true"
else
    print"false"
end

>>true

Solution

Use a different variable name and a actually compare whether they are equal when iterating. Also we will want to use tostring on plr.UserId since the indexes inside our staff array are strings Also we are using break to stop the loop when they're staff to prevent it from continuing.

Example

local staff = {"39939779", "131226890", "169602215"}

game.Players.PlayerAdded:Connect(function(plr)
    local userid = tostring(plr.UserId)
    local leader = Instance.new("Folder")

    leader.Name = "leaderstats"
    leader.Parent = plr

    local rank = Instance.new("StringValue")

    rank.Name = "RANK"
    rank.Parent = leader

    for i, staffUserId in pairs(staff) do
        if staffUserId == userid then
            rank.Value = "STAFF"

            break
        else
            rank.Value = "GUEST"
        end
    end
end)

To fix this do this:

local staff = {"11", "131226890", "169602215"}

game.Players.PlayerAdded:Connect(function()
    local userid = plr.UserId
    local leader = Instance.new("Folder")
    leader.Name = "leaderstats"
    leader.Parent = plr

    local rank = Instance.new("StringValue")
    rank.Name = "RANK"
    rank.Parent = leader
    rank.Value = "Guest"
    for i,v in pairs(staff) do
        if userid == v then
            rank.Value = "Staff"
        end
    end
end)

Simple