If it is an organised wedding with your travel company witnesses will be provided, usually people from the hotel or associated with the wedding company/church. However you can have your own, so if you wish to make to frinds out there first you could ask them to witness. not sure what you like from your holidays but chose the resort carfefully, Bali varies greatly between resorts, from quiet to very lively. If you want to be a little more out of the way go to Ubud or even further north. Have fun!