Do FAQ:
"Can I share the Unreal Engine source code or tools with others?
You can share the source code or tools, along with any modifications you’ve made, with anyone who is an Unreal Engine licensee who is authorized to access the same version of the engine as yours, e.g. the 4.x.x version number of your installed build."
Ou seja, você pode compartilhar a engine e qualquer modificação que tenha feito nela com qualquer pessoa que também seja licenciado pra usar a mesma versão que você.