Go to the Users tab in the script editor and add the hostmask(s) that you want there. Use a user level of something that makes sense. Such as botcontrol or control or something like that. You can use a numbered user level, but named tends to work well for this type of thing.
For examples of how the masks should look, check any that may already be there, or look up user list or user level in the help file and it should show an example line. I'm not on mIRC right now to check the exact format, but it's something along the lines of:
userlevel: hostmask
I know it can take additional properties, but I don't think you need anything beyond that. Then, in your script, just include the user level. Here's a basic example.
on botcontrol:text:join*:*: { join $2 }
As a note, you can add the user information using /auser (and I believe /guser as well depending what you're doing). You can check the help file on those commands for more information also.