Use the physics service to create collision groups, and create a collision group for the boulder, players and make the boulder group not collide with the player group.
In the tab with the toolbox, output and everything, there will be a button allowing you to open collision group manager. There, you can create groups that interact with each other. Everything in the workspace automatically has the ‘Default’ group. You can create a seperate group for the boulder and for the player there. Then in a server script, when the player joins you’ll need to change the property of each part in their character [CollisionGroup] to ‘Player’, or the string name of the collision group you made. You can either manually set the boulder’s collision group in workspace or use a script.