  We present a preferential attachment growth model to obtain the distribution
$P(K)$ of number of units $K$ in the classes which may represent business firms
or other socio-economic entities. We found that $P(K)$ is described in its
central part by a power law with an exponent $\phi=2+b/(1-b)$ which depends on
the probability of entry of new classes, $b$. In a particular problem of city
population this distribution is equivalent to the well known Zipf law. In the
absence of the new classes entry, the distribution $P(K)$ is exponential. Using
analytical form of $P(K)$ and assuming proportional growth for units, we derive
$P(g)$, the distribution of business firm growth rates. The model predicts that
$P(g)$ has a Laplacian cusp in the central part and asymptotic power-law tails
with an exponent $\zeta=3$. We test the analytical expressions derived using
heuristic arguments by simulations. The model might also explain the
size-variance relationship of the firm growth rates.
