Can a creature be invisible and also have an invisibility purge effect centered on it?
Example: A cleric has his sorcerer friend make him invisible and then casts invisibility purge on himself. Does the cleric become visible or can he remain invisible with the other effect radiating out from him?
You surround yourself with a sphere of power with a radius of 5 feet per caster level that negates all forms of invisibility.Anything invisible becomes visible while in the area.
Emphasis added, which gives us a no as the caster is always in the radius of their own invisibility purge spell. The spell does not allow for any exceptions or selective targeting.
I would say it would negate the Cleric's invisibility as well, because of this:
You surround yourself with a sphere of power with a radius of 5 feet per caster level that negates all forms of invisibility.
Anything invisible becomes visible while in the area.
Yes.
At 20th level, a ninja becomes a true master of her art. She can, as a standard action, cast greater invisibility on herself. While invisible in this way, she cannot be detected by any means, and not even invisibility purge, see invisibility, and true seeing can reveal her.
There's another hole in the Invisibility pruge in the form of the Invisible Stalker, which has an ability that keeps it invisible while still being immune to Invisibility Purge. So in the case of this creature, you could cast invisibility purge on it to reveal any other invisible creatures, but the creature itself wouldn't become visible from the spell.