You have two options.

You can scale the dungeon using the group leader. In this case, after the group is formed, pass the crown to the person whose level you want to run BEFORE anyone goes into the dungeon. Keep in mind all dungeons have a minimum level they will scale at. For example, Banished Cells scales down to around 10, while Vaults of Madness will not scale below the lower 40s (i.e., if the group leader is level 23 the dungeon will be in the lower 40s; if the group leader is level 48 the dungeon will be level 48). (You can also scale it to someone in the middle. When I was VR3 we used to scale our VR1/VR3/VR8/VR16 group in Vaults of Madness to VR8 just for fun). Everyone will be their own level in the dungeon, and the drops will not level above the level of the dungeon.

Or, you can battlescale it with the activity finder. If you have all four people you intend to run with, you will want to make sure your roles are marked as having one healer, one tank, and two DPS or you will get an error. You can also only queue for dungeons with a minimum scaling level equal to or lower than the lowest level in your group (although the random dungeon may put you in any dungeon). In this case, the dungeon will be CP160 and everyone will be scaled up, like with the battlescaling in DLC areas. Everyone will get drops for their level.