Assume that a maga wants to do clean herself magically. She comes up with a rego terrem spell that will cause earth to move naturally; i.e., down. The duration is momentary, since the dirt just needs to fall off. The range is personal. But what is the target? Individual and group don't make sense, since there's no such thing as an individual dirt or a group of dirts. But anything else is silly.
Although Personal Range reaches things the caster is carrying, you are not including the caster, so the correct range is Touch (indeed your correct suspicion that T:Group is involved gives the lie to R:Personal);
likewise the many assorted patches of Grime make it Group (there is such a thing as a Group of "pieces of dirt" - certainly magically - as the bits of grime are all similar):
ReTe 4 (GL 1 "move in a natural way (fall off)"*, +1 Touch, +2 Group)
(*if you think Grime naturally attaches, then increase to GL2 "slightly unnatural movement" and ReTe 5.
I would say Group. You are affecting a very loose collection of various types of earths and debree. It's all the dirt that's on you, so that's distinct enough to be a group rather than just a collection, but it's made up of disparate and seperate parts so it's not an Individual.
A comment if I may...
I make the 'assumption' you are doing this as a replacement for the bathing process...???
If you use Terram (dirt) to clean said Magus, the person in question would still likely be quite filthy...
There is likely various things attached to the skin that is NOT Terram. Skin, hair, blood, feathers, sweat, cloth fibers and finally, dirt.
Would it not be easier to change the properties of the skin to allow the foreign objects to fall away??? As you stated, the duration would be momentary...This would also reduce the range to self...unless you wanted a touch spell...
The spell comes out at level 4. Adding requisites for all the other types of dirt around is unlikely to make it uncastable, even for a maga straight out of apprenticeship. 
The individual target for terram is an amount of dirt one cubic pace in size. I believe that the dirt on an individual could arguably be construed as a single metaphysical entity (this entity status being to, my understanding, the litmus test question to discern between group and individual).
If the dirt came from a single event I'd certainly let it be treated as individual.
Well, I certainly get a feel of authoritativeness (thanks!), but I'd like to back that with clarity.
As Erik Tyrell points out, about 27 cubit feet of dirt counts as an individual. Does that quantity of dirt become a group by being spread out over the body?
But about the touch range: personal applies to a person and the clothes he's wearing. Is the idea that, if you want to cast a spell on your clothes, it's target group, while if you want to cast a spell on yourself AND your clothes, it's target personal?
Yes. It's not got any sort of metaphysical, or even physical, unity. Dropped food, mud from the road, and sweat don't have a great deal in common. There's enough to make them a single group, but not, I think, a single individual.
Right. Personal always affects you, in addition to your clothes and such. I might actually allow this as Personal, on the grounds that it's cleaning the maga, and thus affecting her. A spell to change all the dirt on a maga's body into gold would definitely be Touch, however.
Could you get the same effect out of CrCo spell? To clean is certainly to "improve" a body (possibly the same guidline as "preserve a corpse"). It will probably not clean your clothes though. Also it may be a ritual? 
Something like:
The Wizards Bar of Soap.
CrCo 2 Ritual
R:Pers, D: Mom, T: Ind
Cleans the grime off a body in such way that the maga feels that she's taken a refreshing bath. The grime on the clothes stays.
It might be useless, but who knows, sometime in the future (or in the right tribunal) vis might be less worth than silver!
HOT
Well, that is the one of the very few useful applications of target Part:
affect all the clothes a magus is wearing. Therefore I would allow the following:
ReTe 2 (GL 1 "move in a natural way (fall off)"*, +0 Personal, +1 Part)
(*if you think Grime naturally attaches, then increase to GL2 "slightly unnatural movement" and ReTe 3)
I agree, 'Part' is definitely the target in the spell. The dirt is a part of you "You're Dirty" in as much of a way as your morning drool is.
However, I even more agree that the spell is perhaps being made backwards. You're trying to clean yourself (Co), not neccessarily 'of' anything, just clean. I'd have personally done it as a MuCo spell (Minor ability of dirt repellant) with a Terram requisite maybe. Make yourself the target of the spell, and just get rid of the dirt. Base: 2, +0 Target, +0 Range, +0 Momentary, +1 Terram. Then add it to the Level 3 spell list.
Sphynx
Well, I want to discuss the application of target Part in relation to a a person with clothes (Target: Individual) and not your or mine state of cleanliness at certain times of the day. 
Therefore I think the dirt, as a spatial object that can be removed by washing, is target Part.
If you are referring to a state of mind, target Part is clearly not applicable, but washing won't help either. 
If you are referring to a state of mind, target Part is clearly not applicable, but washing won't help either. 
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Ouch..... 