The main pressure points are your index finger and thumb, because they hold the paddle. The fingers on the grip are just holding it in place and should be just tight enough to keep the paddle from moving around in your hand. So yes, you can hold it loosely.
How tight you have to hold it depends on your play style, how comfortable you are with it and which stance you're playing with (the pressure points move a bit in forehand or backhand grips when the wood doesn't fit exactly between index finger and thumb anymore).
If you're just learning the shakehand grip you can simply put a small coin under the tip of your index finger and play without losing the coin. This should give you a proper feeling of how tight you need to hold your paddle. And it helps to keep the finger where it belongs.