You've clicked on an action in the highlighted column and now you're in the Daily Actions page for the culture track. You see your starting vessels displayed on the left in green and your ending vessels on the right in orange. The action group is the set of vessels/wells that you are going to apply an action to. To set up an action group you click,
'Use all available' If the you're setting up the action for all of the vessels in the track.
'Select specific' if you need to pick and choose the vessels/wells for the action. From here you click on the vessel(s), select the wells and save changes.
If you're done, click on 'Save Action Group, or if you need to create more action groups, click 'Save and add new'.
'Use all unaddressed' will create an action group with all of the vessel/wells that aren't in an action group.
Note: Any vessel or well can be part of multiple action groups. A vessel/well can be fed in one action group and treated with something in another, and used to isolate clones out of in a third.