This has happened to someone within our guild at least two different times that I know of. The first time it happened my guild went on a HotW run and I couldn’t zone into the instance with the rest of my party. We tried zoning out of the instance, me zoning out of the zone, returning, going into the instance, reinviting everyone into the party. When that happened, our guild leader couldn’t get into the instance. After trying to get us all in for about 10 – 15 min’s later he dropped party we grabbed a random in the zone and the random was able to join us.
Last night it happened again. We created a guild party, zoned in and one person wasn’t able to zone into the instance with us. Tried everything we’ve tried before and they couldn’t join us. Haven’t heard of this happening to anyone else.