(See the best response by GhostAnime.)
Problem description:
hello I come here because I have doubts about how I can do that only beta testers can enter the game.

and it then appears the window "options and messages" and then appears a message like "Only the beta tester can enter"

and I tried but can not find the way I hope you can solve my doubt
There are a lot of ways to do this but the easiest way is to just make a long if statement with all the names you want to give access to, and under the else condition, have it print your "You can't come in" message to the usr and delete them from the game.
Best response
As EmpirezTeam mentioned, this is simple.

You can do this check either in client/New() or modifying the world/IsBanned() procedure (see the example).

The best way to have a beta list is... well, to have a list()
beta_list = list("Xd!@", "LOL", "U Mad brah? ;=]"

if(!(KEY in beta_list))
Person is NOT a beta member
In response to GhostAnime (#2)
You forgot the ")" at the end of your list.
beta_list = list("D-Cire", "DarkW0lf", "GhostAnime")

usr<<"Beta Testing"
del usr
ooo thank you all for the help xd