All Houses have access to Covert Operations, via the link on the main menu. Full details on covert operations, agents and their uses can be found in the main Covert Ops section of the rules.
If you wish to specialise particularly in the Covert Operations sphere, you may do so by opening this as an additional (or even your primary) sphere of activity. Only by doing this will you be allowed to increase your covert operations budget beyond the level of 'Small', and thereby field a greater number of agents. Having Covert Ops as a tertiary sphere of operations will allow you to set the budget to medium, having this as your secondary sphere will allow you to set the budget to large, and if Covert Operations is your primary sphere, you will have no limit on the size of budget.
In addition to the ability to set a higher covert operations budget and thereby field more agents, any House with Covert Operations as a primary sphere of activity will benefit from the resulting culture of subterfuge - all newly activated agents will be of 'standard' experience, rather than 'rookie'.
Note that should you operate in this sphere, that fact will remain hidden from other players. The sphere will not be shown on your HQ page when another House clicks to view your House details - only you will see the sphere on your own HQ page.
As seen in the Rulebook
Usually what players have discovered about the game that isn't stated in the rules.