My serveur is a 999/90 max lvl but i don't want to give an instance just for player that are max lvl. I'd like to give my player the choice. 
 
They can go in the instance specially made for 99/70, and only player that are 99/70 would be able to register in this instance. Let's call it Blue Instance. 
 
But, Blue Instance would also change depending of the number of player being in the party. 
If there are 4 or less players in the party, then they can register to the Blue Instance : Normal mode. 
But, if they're more than 4 people in the party, of course it will become too easy for them to complete it, so they wouldn't be able to register to the
Blue Instance : Normal Mode. Instead, they would only be able to register to the Blue Instance : Hard Mode. 
 
Same thing with an instance that would only be accessed by party with people lvl 999/90. Let's call it Red Instance
 
IF there are 1 to 4 player in the party, then they can register to the Red Instance : Normal mode. 
But if there are more than 4 players, then they can't register to the Normal Mode. Instead, they can only register to the RedInstance : Hard mode. 
 
So we're talking about
Blue Instance : Normal Mode (99/70, less than 5 players in the party)
Blue Instance : Hard mode (99/70, need 5 or more players in the party)
Red Instance : Normal Mode (999/90, less than 5 players in the party)
Red Instance : Hard Mode (999/90, 5 or more players in the party). 
 
All those instance would give a reward, once finished. Those instance also have a cooldown so that people can't spam it to farm rewards over and over. 
 
But, there would be another option, going in the instance but in a "Training Mode". 
Meaning that in this mode, there is no cooldown at all, they can spam it as much as they want BUT, there is no reward.
 
So, if i'm in a party of 6 players and we're all 99/70, the only instance we can do is the Blue Instance : Hard Mode. But once we've finished it, If we want to do it  
once again we can but only in Training Mode (but they won't be any reward). 
 
And there would be a ranking for every instance, so Blue Instance Normal, Blue Instance Hard, Red Instance Normal, Red Instance Hard, Blue Instance Normal Training (no reward no CD), 
Blue nstance Hard Training (same), Red Instance Normal Training (same), Red Instance Hard Training (same again)