Can I let other travel agents sign up on my portal and sell through me at rates I decide for them?

Yes, that is a normal way to use a white label portal, and it is usually called running a sub agent network, though how far you can take it depends on the plan you are on and on what your suppliers permit.

The idea is simple. Another agency registers on your site, you approve them, and when they log in they see the same search results your own customers see but at the rate you have set for them rather than your public selling rate. You can group agents so that one set gets a better rate than another, and you can keep your public rate untouched for walk in customers. Every booking they make sits under your account with the supplier, so the supplier still sees only you.

There are limits worth knowing about before you build a network. You carry the payment risk, because the supplier will look to you when a booking has to be paid for, so most agencies either take card payment from the agent at the time of booking or give a credit limit they are comfortable losing. Some supplier contracts also restrict who may resell their rates, and a few products such as certain negotiated fares are not meant to move down a chain at all, so it is sensible to ask rather than assume.

Start by writing down the rate levels you want to offer and how each agent will pay you, then check your supplier agreements for any resale wording before you invite the first agency in.