The best way is if they developed some sort of symbiosis with them. Over time in confined areas reptiles have developed symbiotic relationships with birds. You just need to be creative on how it happened plausibly with your tribal people.
Tribal people domesticated all the animals we have today, so that bit is not an issue. We can tame some reptiles but it's a lot of work and best done with captive born. So it is possible to some extent.
But you're talking about huge monsters, so taming them as we do reptiles isn't an option because even tame reptiles will take a bite every so often if they feel like it. A T-Rex would make a messy bite, you couldn't just wear stout gloves and expect to get away with just a bit of a scare and perhaps an involuntary bowel movement.