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An action group is (visually) just an action that has other actions inside of it. So if you just typed "Weed and Feed" as an action, type return, and then command-]. This will create another action and indent it under "Weed and Feed". Now you can name this "Buy Weed and Feed" and type return to create "Spread Weed and Feed". Another return and command-[ will make the next action outdent, so you can type "Mow Lawn".
You can also just list out all the actions and then drag one up and to the right a bit, until a black right arrow appears, and that'll indent it.

Once you've got actions under an action, it becomes an action group. Then you can change it from parallel to sequential or vice versa in the top section of the group inspector.