Thats actually something that has been bothering me for a while, having party raids. Then again, having party raids for a daily raid, doesn't add up. Simply makes it too easy to overcome. So instead of entering with party raids. Either limiting it to 5 instances a day with maximum 2 party or once party raid per week. Unlike WoW the content in hand is limited. One of the possibilities could be spreading out dungeons that open up only if the instance is acknowledged. For example ; A new dungeon has opened up in Route 10, you're given the task to investigate the ongoing uprising among pokemons that have taken residence in there.
This would require complex scripting and coding. And needs to be thought out carefully without effecting anyone elses gameplay. I.E Rock tunnel.