It depends. As part of their contracts, stars typically get a pair of comps for every performance (as do each member of the creative team -- some get more than a pair). If they can't use them for any given show, they can turn them back in to be sold (or in the case of a long run, they may simply waive their comps after they've run out of family and friends to give them to -- they, of course can always request more house seats at some later date if they need them).
Non-stars typically can access house seats as they need them, but they may not be free and must be paid for either by them or their guests.
"What a story........ everything but the bloodhounds snappin' at her rear end." -- Birdie
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"The Devil Be Hittin' Me" -- Whitney