If you deploy an action/patch to a group, then add additional machines to the group, do the new machines added to the group also become relevant for the action/patch and therefore run the action/patch?
In some limited testing I did, the new machines added to the group did not seem to pick up the action/patch even though they should have been relevant for it.
Yes, you can target an action to a group, and computers added later will still become relevant.
However, there is a big difference in how you “Target” computers in the “Take Action” dialogue.
Make sure that in the “Target” tab in the take action dialogue, you select the second radio button “All Computers with the property values selected in the tree below”, and then select your group.
If you had selected the first radio button “Specific computers…”, then effectively, you are only targeting the computers listed at that point in time.