You can get married here on tourist or any kind of visa, and after that you can apply for one year stay in Montenegro by certificate of marriage.... you will need some documents from your country like certificate of birth, proof that you are not married before (depending how this documents looks like in India you can bring it from there and make translation here by official translator or to make in embassy here, if there is no Indian embassy here you will need to go to Belgrade)... after getting marriage certificate you will jut need passport and some papers to be done here to get this one year visa which you can extend every year, and after 5 years you can get permanent stay, and later citizenship......
anyway ask your darling to check something, but this is 100% right