I need to know how to have a list(var/list/Testers = list("Chwgt")) saved. And then when they login, it check the list for their key if it's there, they're allowed in, otherwise they're booted. And a verb to add testers while in the game(Not sure if mines messing up or if it's something else).

Any help is appreciated.

Usually it helps to show us code. Yeah, use if(src.key in Testers) that should work.
Simple enough.

testers = list("chwgt")
// save the 'ckey' version of their key

var/savefile/S = new("testers.sav")
S["testers"] << testers

var/savefile/S = new("testers.sav")
S["testers"] >> testers

if(!(src.ckey in testers)) // if their ckey isn't in the list, don't let them in
src << "You are not a tester, so you cannot log in now!"
// regular login code

verb/add_tester(t as text)
t = ckey(t)
if(!t) return
testers += t
src << "'[t]' added to testers list."

Just call save_test_list() when the world is closed, and load_test_list() when it is created.

~~> Unknown Person
In response to Unknown Person
In response to Kalzar
This is Developer How-To. If the post is in the right position, and it is, we need not any code. He's actually asking for code.
In response to CaptFalcon33035
Yeah, I read his post wrong =P