They are making tiny weird Ashran changes every week. Currently you can't even bring in 5, but I have been successful with 2 of my team getting into the same. While Hollinka does say he thinks its fine to bring 5 people in, If you read further down you see him say how some people were bypassing it and they had to "create additional measures".

Right now the only way is to enter Ashran, AFK in base for 5 minutes, then leave and re-enter. Eventually everyone is in the same instance, takes about 30 minutes.

But wait until next week and it will be changed again. They are not putting any developer resources into previous content with Legion on the horizon, but these weird tiny changes keep coming non-stop.