startup.lua

local users = {Relic = "password", Bob = "cow"}
print("enter existing user: ")
local user = textutils.serialise(users[read()])
if user=="nil" then
    print("That user doesn't exist")
    os.sleep(1)
    os.reboot()
else
end
print(user)
print("Username: ")
local inputuser = read()
print("Now password: ")
local inputpass = read()
table.insert(users,textutils.unserialise(inputuser).." ".."=".." "..inputpass)
term.clear()
print(textutils.serialise(users))