Yes, sub agents can register themselves through a signup form on your B2B portal, and you decide whether they go live straight away or sit in a pending list until you approve them. Most agency owners choose approval, because it lets you check who you are dealing with before they can book anything in your name.
The signup form can ask for the things you would normally ask for on the phone, such as the company name, the owner or contact person, the country they operate in, the currency they want to trade in and a copy of their trade licence or travel association membership. Until you approve the account, the agent can log in and look around but cannot confirm a booking. Exactly which documents you can collect and whether they upload them directly depends on how your portal is set up and on the plan you are on, so confirm that before you announce the signup page.
When you approve an agent, you are also setting the terms they will trade on, such as which markup group they belong to, whether they book on prepaid balance or on credit, and which products they are allowed to sell. The system can hold all of that, but it cannot judge whether an agent is trustworthy or whether their licence is genuine, so that check stays with you. It is also worth remembering that some supplier agreements limit who you are allowed to resell to, so your own contracts may decide who you can take on.
Decide first whether you want approval to be manual or automatic, then write down the documents and details you want every new agent to give you, and build your signup form around that list.