Since version 3.2 you can create your own libraries. These are special scripts that can be included from other scripts so you can share the same code across multiple scripts. Use special keyword include to include another script. Provide script title as parameter to include needed script. Here is example:
include "common.lua" function main(userId) myCommonFunction(); end
Best practice it is to put include outside any function calls. Please note that global variables in libraries will not be shared across all scripts. Only code will be shared.