For an example let's use Chorus of the Righteous (NP pg 97).

"The psyker chooses a number of allies (excluding himself) up to his Psy Rating within range. As long as the psyker sustains this power, each of these allies gains a +5 bonus to Weapon Skill and Ballistic Skill Tests, with an additional +5 bonus to these Tests for every Degree of Success the psyker scored on his Focus Power Test (to a maximum of +30)."

So let's say he casts at Psy Rating 6, so range is 12m, and then he moves back 2m, making ally 5 out of range. Does he keep the buff or lose it? If he loses it can the Psyker choose someone else to gain the buff? If he comes back in the circle does he get it back?

Is there RAW rules on this somewhere?

As per the basic Psychic Power rules on page 158 of Rogue Trader Core Rulebook an individual or group of individuals must be "inside the power's stated range" to receive any effect. As such, the allies would have to remain within the 12m radius or lose any buffs gained from it.

The psyker would not be able to assign a new ally as the target, as that would have been done during the casting process.

However, the target would per RAW regain the buff if the entered the radius again during a subsequent turn.

Alright thanks for the info.

Also there's no downside to just repeatedly casting it on Fettered level until you get the 6 degrees of success if you're preparing for combat right? That seems majorly broken. You can even use spam invocation to boost your fettered power rating.

Not anymore broken than any buff spells from D&D or similar systems. Also, if your players are all bunching together like that... shoot a missile at them. Let them dodge a Blast (5) or higher Frag Missile wielded by a random enemy mooks. Because enemies have heavy weapons as well.