This page contains biographies of the current Assassins. And like on the Thieves' page, there's a page linked at the bottom of now-dead members of the Guild.
BELLA DONNA BOUDREAUX: Current viceroy of the Unified Guilds, former matriarch of the Assassins Guild and ex-wife of Gambit, Bella Donna made her first appearance in X-Men #8. She is a telepath, although I do not think she's a very strong one, and she's a very skilled killer.
GRIS-GRIS: This huge (yeah he's a big man) voodoo master first appeared in the Rogue Limited Series. He holds the Assassins Guild title "First Kill" and was right-hand man to Marius Boudreaux before Marius died, and is now what Bella Donna calls her "besider". He has the ability to manipulate emotions.
SINGER: This beautiful killer first appeared in the Rogue Limited Series, just like her male Assassins Guild counterparts. She can fly (in fact I have never seen the woman walking anywhere yet!) and is also a communications channeler. She usually uses that power to project messages (through her eyes) from Bella Donna to other people.