If you pack the bed of the truck properly nothing is going to go flying anywhere unless it's super light. I imagine with 5 guys you will have it packed fully so no worries. If there is anything you absolutely cannot get wet I'd just use a contractors style trash back and cover the items up and tie it off.
For peace of mind you can get a cheap cargo net from Harbor Freight or your local auto supply store, think $15-50. I've never used a tarp to cover stuff but if you must I'd feel much better having a cargo net on top of the tarp to prevent it from blowing around.
I'm always weary of people stealing my shit when traveling with my truck so if you hit a rest stop or grab food sit somewhere within sight of the truck or have someone stay out and keep an eye on things. (When I haul motorcycles/gear I'm paranoid and always have a cable lock running through everything for peace of mind. Not sure that'd be suitable for this situation though.)