As it says "centered around a creature" I would take that to mean that only the target would be entrapped. Of course, if he shared his square with a familiar or possibly a mount then I would allow that creature to possibly be entrapped as well. It would just depend on the circumstance and the size of the sphere.
However, I would not allow it to pick up the wizard standing next to the fighter if my intended target was the fighter.
1-ft.-diameter/level sphere, centered around a creature
So at caster level 10 it is a 10 foot diameter sphere, which is large enough to cover 4 squares. Which means it has enough room for more than one creature.
so how does it develop?
A sphere-shaped spell expands from its point of origin to fill a spherical area. Spheres may be bursts, emanations, or spreads.
This suggests that anyone near the point of origin (namely the target) will be pushed out of the area.
However:
The sphere functions as a wall of force, except that it can be negated by dispel magic.
Which is a wall spell which develops at the perimeter of an area. Which suggests that instead of the normal push out it could simply be a literal round wall of force (that is susceptible to dispel magic) meaning it could surround more than one person.
BUT:
A globe of shimmering force encloses a creature, provided the creature is small enough to fit within the diameter of the sphere.
Hints that it will only contain one creature...
So what happens when a creature is contained and starts summoning creatures that would fit in the space inside the sphere (since you have to have line of effect to where the creature will appear)?
Personally I have ended up treating the spell like a wall spell and allow multiple creatures to be targeted if they all fit into the space the spell will cover. Since the spell allows a save throw I generally go with the following line from wall of force
If its surface is broken by any object or creature, the spell fails.
and say that if a creature saves they successfully imposed themselves and broke the surface as the spell was forming and prevented the spell.
Effectively meaning that if you do not contain a creature with it you cannot use the spell as a means of area denial.
So what happens when a creature is contained and starts summoning creatures that would fit in the space inside the sphere (since you have to have line of effect to where the creature will appear)?I'm beginning to think that the sphere is full, and not just a shell.. This way the creature contained would not be able to move and there would be no space for other creatures. If this was the case the push out thing seems a bit more reasonable no?
That wouldn't follow with the wall of force spell though.
The size is variable based on your CL. You can always choose to not use the full benefit of your CL to a spell effect. The spell requires that you only enclose one creature with it (even if you have the potential space for more).
Answer seems pretty clear to me... The sphere stops expanding if going further would include another creature inside it.