Lets pretend there is a unit of skellies that is completely surrounded on all sides in combat. It is 25 models in a 5x5 formation. Each of its four sides is engaged by units that are 5 models wide in the front with the same base size. Very unlikely I know, but it has a practical question behind it. Now the VC player casts Invocation on the unit, can he add more models to it, or no? Imagine that it is the same situation, but the back rank of the skellies isn't complete, say there are 23 models in a 5x5 formation, so that there are two empty spaces in the back rank, Is the VC player limited to summoning only 2 models? Or can you summon as many as you want and you move the unit engaged in the rear back if possible?
This type of thing has come up, mostly if a unit of undead is engaged in the front and rear at the same time. We were unsure if we should fudge it and add models as long as it didn't interfere too much with something else in game, or if we should say that the unit can never be summoned into again in effect, since the new models had nowhere to go.