It depends on who you ask. Currently, two different offices at Disney are providing diametrically contrary information about that. The folks responsible for answering questions guests pose, like that, say sharing Dining Plan meals isn't permitted. They'll provide that advisory in writing. OTOH, rumor is that folks at Disney Dining's offices will verbally tell people that it is okay to share meals (and, of course, servers are allowing it in most, but perhaps not all, cases), as long as you're not sharing with anyone not on the Dining Plan. They won't, however, put that in writing, at least not so far.