Are you even permitted to have an M&P 15 in California?
Just kidding.
Depends, does California permit assualt rifles? Don't think so. If not then is seems seems California doesn't permit a lot of things, a forward grip on the M&P15 is one of them I think. Unless you do not have a detachable magazine, then a grip would be permitted it seems from a simple reading.
California Penal Code Section 12276.1 - California Attorney Resources - California Laws
(a) Notwithstanding Section 12276, "assault weapon" shall
also mean any of the following:
(1) A semiautomatic, centerfire rifle that has the capacity to
accept a detachable magazine and any one of the following:
......
........
.......
(F) A forward pistol grip.
see also:
http://ag.ca.gov/firearms/dwcl/
(Also for handguns, see >
http://certguns.doj.ca.gov/)
California 2008 Dangerous Weapons Control Law >
http://ag.ca.gov/firearms/dwcl/12275.php
So even though section 12276 lists specific weapons that they class as assualt weapon, and the M&P 15 is not among them, it does list "(5) Colt AR-15 series" and further down it says "(e) The term "series" includes all other models that are only variations, with minor differences, of those models listed in subdivision (a),
regardless of the manufacturer." so that might include the M&P 15 if not modified to california standards (fixed magazine for example), don't know for a fact. However, in section 12276.1 it says that assualt weapon "shall also mean"....
so, assuming your M&P 15 has been 'Californicated' and your magazine is not detachable then its not an assualt rifle under section 12276.1, but the question of section 12276 remains, but if section 12276 does not apply then under section 12276.1 you can have a grip if you do not have a detachable magazine... if they do not class the M&P 15 as part of the series of the Colt AR-15 type. California law is so confusing when it comes to firearms so don't take my word for it, i'm not an attorney. If it were like Tennessee, the paton state for shooting stuff, then you would have no problem, but California is a whole big mess on gun laws. Have you tried contacting a local reputable gun dealer and asking? They should know.