I read this on reddit and got a chance to test it when I joined a 2-man LFG party that was stuck on the 32 end boss (with changing shields). If you try to join someone in the 5th round it probably won't work -- it gives you an error that says the target game is unjoinable at that stage. However, if you join at exactly the right time it will work.

1. Have the in game people wipe.
2. Spam the "join fireteam" button.
3. Repeat until it works.

It didn't work the first 10 times I tried (during their first wipe), but it worked the first time during their 2nd wipe.

Edit: Others have reported that you can join PoE level 32 in progress on rounds 1-3 without using this technique.