  A new network evolution model is introduced in this paper. The model is based
on co-operations of $N$ units. The units are the nodes of the network and the
co-operations are indicated by directed links. At each evolution step $N$ units
co-operate which formally means that they form a directed $N$-star subgraph. At
each step either a new unit joins to the network and it co-operates with $N-1$
old units or $N$ old units co-operate. During the evolution both preferential
attachment and uniform choice are applied. Asymptotic power law distributions
are obtained both for the in-degrees and the out-degrees.
