Williams had been widely expected to make himself available for the sevens team for Rio after his switch back from rugby league late last year.
Messam, a former sevens player before he made the All Blacks, had also said he was keen on trying to win Olympic gold in Rio with Gordon Tietjens' sevens side.
"We know what Liam can do and he will certainly give us some new options up front," Tietjens said in a statement.
"He's a hugely experienced sevens player having won two Commonwealth Games gold medals and been part of sevens World Series winning New Zealand teams.
"Sonny Bill Williams is a great athlete and has shown he can adapt very easily to the demands of rugby.
"I see him adjusting very well to the fast pace of sevens. He'll be a great attacking weapon with his speed, strength and uncanny ability to offload."
Neither player is expected to be involved in Super Rugby next season for the Chiefs due to their sevens commitments, where they will be expected to play at least four of the 10 tournaments on the World Sevens circuit.
Tietjens said the pair had agreed to be available for the entire series.
Several other high profile union players, including All Blacks Julian Savea, Beauden Barrett and Ben Smith, had also expressed a possible interest, though Smith and Barrett said last week they wanted to concentrate on the 15-man version.
Barrett's Hurricanes team mate Ardie Savea, a potential All Blacks openside flanker next year, had also been named in the squad, which will form the basis of their teams for tournaments in Dubai and Cape Town in December.
Other players outside the squad would still be considered for other tournaments, Tietjens said.
Players named in Super Rugby sides would play the Wellington and Sydney tournaments in late January/early February then return to their franchises before playing the final two tournaments in Paris and London in May.
Scott Curry was named as captain, replacing long-standing skipper DJ Forbes, who wants to concentrate on making the Olympic team on form rather than being chosen because he was captain.
Squad: Scott Curry (captain), Dylan Collier, Sam Dickson, DJ Forbes, Gillies Kaka, Akira Ioane, Reiko Ioane, Ben Lam, Liam Messam, Tim Mikkelson, Ardie Savea, Sherwin Stowers, Sam Vaka, Beaudein Waaka, Joe Webber, Sonny Bill Williams.
