On my last flight, an Early Bird saved 5 seats in addition to their own. When I wrote Southwest about this, I was told that's perfectly fine to save seats for someone who "may" (or may not) be boarding later. So presumably you can prevent that middle seat from being taken just by claiming you're saving it for a friend in C group who "may" be boarding later. Early Bird should entitle that individual to one seat only, not as many seats as can be roped off.
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