What instance are you refering to here? As in world phasing? Invasion points?

I had no real issue keeping my 3x parties of 5 in the same world phasing. Start of the expansion was a bit worse, even with less toons, but now they more or less always ended up on the same shard. This might be realm dependant though.

As far as invasion points, I found it took some convincing at times. the most consistantly guaranteed method of forcing the same instance was dropping my groups outside of the portal, taking 3 toons in, checking they were in the same instance and having all of them invite 4 others and having the rest enter.