Harper agreed to a 13-year deal worth $330 million with the Phillies on Thursday. Bell posted a screenshot of a message sent from Harper on Instagram after the running back congratulated the slugger on his signing.
Harper's message included an invitation for the upcoming NFL free agent to "come join" him in Philadelphia to "bring as many titles as we can to the greatest city."
"Aye come join!" Harper said in the message. "Let's take this city by storm. Bring as many titles as we can to the greatest city in the world. Brotherly love. #SecureTheBag."
Bell will enter free agency when the new league year begins March 13. The three-time Pro Bowl runner sat out of the 2018-19 campaign instead of playing under the franchise tag for a second straight season.
Sources told ESPN last week that the Steelers will not use the franchise tag on Bell again, meaning the 27-year-old tailback will be an unrestricted free agent.
The Steelers drafted Bell in the second round of the 2013 NFL Draft. He is a three-time All-Pro selection. In five NFL seasons, he has 1,229 carries for 5,336 rushing yards and 35 scores. He has added 312 receptions for 2,660 yards and seven touchdown catches.
Harper will join the Phillies after posting a .249 batting average with 34 home runs and 100 RBIs with the Washington Nationals in 2018.