Molasses is simply the yuck left over from processing cane sugar. Traces of molasses are what makes brown sugars like Demerara, Turbinado and ... er ... brown sugar, well, brown.
That sentence got completely out of control, didn't it?
Anyway, molasses is not completely fermentable. The impurities will leave flavors. I recently made an historical porter recipe which got some of its color from a large amount of 'essentia bina', which is homemade molasses, in the boil. The beer is, shall I say, an acquired taste.
If you prime the bottles with molasses, you'll get a definite but not overpowering flavor contribution. Use the same amount you'd use of cane sugar for priming.
Bob