The concept of angular momentum can easily be extended to include particles, constituting a system. Each of the particles can be associated with certain velocity. In angular parlance, we can assign angular momentum to each of them, provided they are moving. They may be subjected to internal as well as external forces. This is very important difference between a "particle" and a "system of particles". When we consider a single particle, the force can only be external. A particle occupying a point of zero dimension can not be associated with internal force.
For visualization, we can again consider the example of particles or billiard balls that we used in the context of linear momentum. The situation, here, is same with one exception that we shall refer to a point (origin "O" as shown) for calculating angular momentum as against calculation of linear momentum that does not require any such reference point.
Angualr momentum of asystem of particles  


The angular momentum of a particle in three dimensional space is defined by the vector relation :
where “r" and “v” denotes the position and velocity vectors respectively. We can combine angular momentum of one particle provided angular momentums are measured about a common point. Though, we can calculate angular momentum about different points, but then no physical meaning can be assigned to such calculation. This requirement is actually the reason that angular momentum, in general, is defined about a point  not about an axis. It would have not been possible to associate motion of particles with a common axis.
Since angular momentum is a vector quantity, the angular momentum of all the particles is equal to the vector sum of all the individual angular momentums. It is imperative that the summation would require application of vector addition rules to get the sum of the angular momentums.
The angular momentum of the system of particles is denoted by capital “L”. The particles may change their velocities subsequent to collisions among themselves (due to internal forces) or because of external forces. Consequently, angular moment of the system may change with time. The first time derivative of the angular momentum of a particle is equal to the torque on it :
The most critical aspect of this equation is that the sum of torques involves both internal as well as external torques. When we talk of torques on the system of particles, the classification of internal and external depends on the boundary of closed system. This, in turn, depends which of the particles are included and which of the particles are excluded from the system. The forces (constituting torques) applied by particles included in the system constitute internal torques. The remaining ones are external torques. A particle can not have internal torque anyway. We must also understand that when we say a torque is applied on a particle (without any qualification), we implicitly mean that torque is external to the particle.
According to Newton's third law, the internal forces (torques) appear always in the pair of equal and opposite forces (torques). As such, they cancel each other. The expression of angular momentum, then, reduces to :
where "
This relationship is same as that of a single particle. Only difference is that net external torque, now, is equal to the first time derivative of the vector sum of angular momentum (L) of the system of particles as against a single particle. Evidently, this relation holds for measurement of angular momentum and torque about the same reference point.