sign with (someone or something)
To sign a contract of employment with a particular group or organization. Used especially in reference to professional athletes. The star quarterback has signed with the Miami Dolphins for the next season. I'm planning on signing with the law firm in California, unless the firm in New York offers me a better deal.
sign (up) with someone or something
to enter into an agreement with someone or a group. I signed up with Tom and John to crew their ship in the regatta. Did you sign with the office equipment supplier yet?
To sign an employment contract with some organization: The soccer player signed with a team in Spain.