How to add a modifier to a collection?

Question

My goal is to reduce the faces of an object using the decimate modifier. The object I'm editing has lots of vertex groups. Blender 2.8 only lets me add the decimate modifier to a single group at once which would take quite some time. I have already put all groups in a single collection.

How can I add the decimate modifier to the entire collection?

Do I have to add the modifier group by group?

Answer 1

You have to join the objects together and then apply the modifier and then you can separate by loose parts.

Or

1. Add the modifier to one object.
2. Select all the other objects and then the one with modifier last.
3. Then Ctrl+L to copy modifiers to all selected objects.

Answer 2

As there are 2 options to copy modifiers from one object to another or multiple objects, I wish to point out the difference that is not so obvious when using either.

To use either method, selecting all target objects and having the object with the wanted modifier as active object (selected last, brighter outline) is needed.

1. Copy Attributes (Add-on) default enabled offers the Ctrl+C menu with the options to copy various types of attributes like location, rotation, object color and also modifiers.
   
   
   There is a big difference between the marked 1. Copy Modifiers and the 2. Copy Selected Modifiers. The first one will overwrite the already existing modifiers on the new target-objects, while the latter will add the selected modifier from the active object to the target-objects modifier-stack(means below/after already existing modifiers).
2. Make Links menu opened by pressing Ctrl+L.
   
   Here choices regarding linking are given one of which is Modifiers. The option here results in all modifiers from the active object being copied to all the target objects. While doing so, the target objects will lose all their eventually pre-existing modifiers, as the operation overrides the modifier stack with the one from the active object.

That said, both options do copy or overwrite, not link the modifiers, as modifiers are not the same as mesh-data that can be shared by objects. So after modifiers are copied, each object has their own modifier-stack which are not linked and thus do not affect each other in either direction like linked copies of objects would.