How?
Without weapons and training, the only way they can assist is by facilitating the invading forces by providing them lodging facilities, food and shelter(esp useful for special forces). Even if weapons are provided on an emergency basis, not much can be expected from untrained people or people with rudimentary training. Maybe, the numbers of invasion troops can be swelled by the induction of local people, but when one can't be certain how they'll react to mortar or arty round (run away or hold fast) so it remains a gamble.
Furthermore, 4-5km is still the mountains, not the valley which is boiling --- the reaction of those people might be different than the reaction of the valley dwellers. We must also remember that Kashmir is not a homogeneous entity, Ladakh has never seen an uprising till now, Jammu remains pre-dominantly Sikh/Hindu region. The valley has a strong support for Pakistan today but it is much deeper than 5-6 km.
Ground needs to be prepared for that.
As we speak, there are only around 400 armed fighters inside IOK, most of whom are lightly armed, lack proper training and seem to be unfamiliar with the intricacies of guerrilla warfare bar a few top leaders like Naikoo. Indian army has more than enough manpower required to crush even a well trained guerrilla force of that number. Unless the LOC is softened and safe heavens plus volunteers provided on this side of the border to initiate a well organized uprising in IOK before we launch an actual invasion, we can only hope for food, shelter, intel and navigation. We can muster 25k-40k armed volunteers within 2-3 months who can be used in IOK as a guerrilla force (based on past estimates, we did have much more than that number of armed Mujahideen from Punjab, AJK and KPK)
In any case other than limited asymmetric warfare, the role of airforce will be the deciding factor. Whoever achieves air-supremacy is bound to win. IMO we should've spent more on PAF and less on PA
We musn't forget that AJK is our first line of defence which protects Pakistan's geographic vulnerabilities in the North; Water from there is our lifeline which is threatened over and over again by India.