The only way around sharding is to get invited to a group that’s already on one shard. You’ll be ported over there and even if you leave the group, you’ll still be on the shard.
At least until you reload the zone.
The only way to do it on a larger scale is with concerted cooperation with multiple people. During the last event in Orgrimmar I participated in, we basically had multiple people on alts parked on different shards -there were at least 3 or so- and whenever we saw someone who looked like they were in the area, we’d advise them to /whisper for an invite.
So while our shard was filled with dozens of people, the others were virtual ghost towns.
I wish instead of automatically shunting you off to additional instances, the game at least gave people a choice of which specific instance they wanted to be in. It’s fairly standard in most other MMO’s I’ve played.
Fortunately, the Diablo promotion ends in a week. I expect the sharding to end then as well.