In the paper they give a wide estimate of the possibility of lithopanspermia, ranging from 0.001 to 1.6 events per cluster, which is perhaps a little less optimistic than the 'few' mentioned in the abstract. I deliberately adopted the optimistic end of the estimate, assuming (for the purposes of fiction) that life emerges frequently.
The number of unknowns are very high. If abiogenesis occurs frequently and early while the stars are still in the cluster then the rate of cross-infection increases. Additionally the cluster can act as a net to catch wandering life-bearing rocks; it seems possible that once a rock wanders into a tightly packed cluster and starts off a process of lithopanspermia in that cluster, several worlds might be infected in the heavy bombardment phases of planetary development in each system. This would drive up the frequency of cross-infection, if the process happens at all.
On the downside there is no guarantee that a lifebearing rock will sucessfully infect the planet it fall upon; the environment in some larger clusters is dangerous because of the hot, bright stars which are prone to explode, and life may quickly die out on many or most worlds even if a biosphere forms.
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