I found something similar, Randia aculeata-- White Indigo Berry in the family Rubiaceae. Yours appears to be either a young plant or it's something totally different, it's hard to say without getting a closer look at the leaf texture etc. The berries look immature.
http://www.rarefloweringtrees....
http://www.shirleydenton.com/p...
http://irrecenvhort.ifas.ufl.e...
Have you seen the flowers? Have the plants been around long?
It's native to Florida, grows 2 to 6 metres tall..
http://en.wikipedia.org/wiki/R...
