Real-time symmetrical modeling with distinct objects in Blender

Question

I would like to create a 3D model that has a wide variety of individually selectable objects on the right side of the x-axis, and then all of those same objects mirrored dynamically across to the other side of the x-axis. How do I do it?

What I want to achieve

1. Create an object with an origin at 0,0,0 and no parts jutting into the left side of the x-axis.
2. Create another object (say, a cube) somewhere on the right side of the x-axis.
3. Somehow create an association between these two objects that lets me still select them easily later.
4. Apply a mirror modifier to the association.
5. Create more objects on the right side of the x-axis and add them to the association so that they are also mirrored on the other side of the x-axis.
6. Add objects, move objects, change objects, and have them update in real time across the axis.

What I've tried - Grouping objects and then putting a modifier on the group (I wasn't able to get this to work) - Ctrl+J to join the meshes (but then I can't select them easily individually)

Answer 1

This is what Groups and Group Instances are for, you just don't add modifiers to them.

Create your selection of objects you want to belong to this symmetry group, then group them together with Ctrl + G. Give it a descriptive name

Now from the Add menu add a new group instance with the previously given name. Scale it a factor of -1 in X so it becomes mirrored.

You may optionally want to make the instance non selectable in the Outliner so it doesn't accidentally get in the way.

Now freely work on your original group of objects and see them symetrized automatically. Whenever you feel like adding a new one just link it to the existing group.

You can do this by selecting any new objects, then Shift selecting one already belonging to the group so it remains the active one. Then press Ctrl + L > Make Links > Group.