What differentiates a "group of rogue stars" from a galaxy? According to Wikipedia, a rogue star or intergalactic star is


  not gravitationally bound to a galaxy.


But according to a citation on the same Wikipedia page, the first intergalactic stars were discovered in the Virgo cluster of galaxies. 


  These stars form a massive group  aproximately 300,000 light years away from the nearest galaxy. (Emphasis mine)


Is there any clear, stated definition which differentiates galaxies from "massive groups of rogue stars"?
It is just a poor choice of words. The comment refers to a discovery paper by Ferguson et al. (1998), where they used a very deep image in the Virgo cluster to establish that there were an excess of distant stars compared with a control field outside of the cluster.

This excess of about 630 stars were all in one tiny HST field of view, so form a "group" in some sense, but there is no suggestion that these stars are not found throughout the Virgo cluster. Indeed subsequent work has found evidence for intracluster stars in other areas of the cluster (e.g. Durrell et al. 2002).