However, Spain’s Jon Rahm has withdrawn from the league, which was launched jointly by McIlroy and Tiger Woods.
Rahm had been one of the first to pledge his support to the venture but now says he is unable to commit.
In a post on social media, he wrote: “I am sad to confirm I will not be participating in the first season.
“Right now it would require a level of commitment I can't offer.”
The league gets underway from January 9 in the New Year and will be overseen by TMRW Sports, which is a technology-focused sports firm launched by McIlroy and Woods in August last year.
County Down man McIlroy, currently ranked number two in the world, said: “I’m thrilled to join Boston Common Golf and to be representing Boston and New England’s rich sports culture.
“TGL brings a unique dimension to the game, and I can’t wait to stand with Adam, Keegan and Tyrrell as my teammates.
“It’s a fantastic opportunity that will offer us all a chance to build lasting connections and to foster regional pride that you don’t often see in the game of golf.”
McIlroy’s team will be backed by Fenway Sports Group – owners of Liverpool FC – and faces opposition from five other teams of four players each.
Each team will play each other once in a league format but only three of the four players will compete in any one event.
So far, Justin Thomas and Collin Morikawa – both two-time major champions – are the only other players to be assigned to teams.
Morikawa will represent Los Angeles Golf Club – backed by tennis stars Serena and Venus Williams – while Thomas will represent Atlanta Drive GC.
Basketball superstar Stephen Curry is investing in the San Francisco franchise, while Steve Cohen, who owns baseball outfit the New York Mets, is head of the TGL New York team.
The sixth and final franchise is yet to be announced.
With Rahm ruling himself out, 14 of the current top 20 in the rankings are set to participate.
The league is scheduled to run for 15 weeks, culminating with the semi-final a best-of-three final.