Dombrowski and Boswell, both 21, join the squad as neo-pros after two seasons at the Bontrager-Livestrong Under 23 team.
Dombrowski was fourth in the queen stage of the Tour of California and won the Baby Giro d’Italia.
Boswell finished second in the U23 Liege-Bastogne-Liege and joined Argos-Shimano as a stagiaire this year.
Team Sky coach Bobby Julich said: “Bontrager-Livestrong - and especially USA Cycling - have done a great job developing these guys into the riders they are today and we want to continue that hard work now to ensure they reach their full potential in the future.
“Ian and Joe are two of the most talented under-23 riders in the world and we will offer them the best possible race and training programmes to aid their progression. We see this as a long-term project at Team Sky and it’ll be exciting for everyone involved.
“We must be doing something right if they’ve chosen to sign for us over the American teams. I followed a similar path during my racing career, went through a lot of the same things they are going through at the moment, and I think we can help them a lot.”