The only way to join an agency is to be invited by an agency administrator. Your relationship with them is managed privately outside of Upwork.
Agency relationship types
Exclusive Agency Relationships
Exclusive agency members work on Upwork only as part of their agency. This is the most common type of agency relationship. Your profile and proposals will be branded with your agency to make the agency’s part in the work relationship clear to the clients. All contracts will be paid to the agency.
Non-Exclusive Agency Relationships
Non-exclusive agency members can work on Upwork independently and as part of an agency. Agency contracts will be paid to the agency. Independent contracts are paid to you.
By default, you’ll appear in the marketplace as an independent freelancer not associated with an agency. If you wish to be treated as an agency member in search results, on your profile, and when clients reach out to you, you’ll need to set the agency as your default team in your Settings › My Teams.
Either way, you’ll have the choice every time you submit a proposal to specify if you’d be working on that project independently or as an agency member.