If you'd like additional users to have access to your organization's private repositories, you can purchase more seats anytime.
In the top right corner of GitHub, click your profile photo, then click Your profile.
On the left side of your profile page, under "Organizations", click the icon for your organization.
Under your organization name, click Settings.
In your organization's Settings sidebar, click Billing.
Under "Billing overview", click Add seats.
Under "Add seats", type the number of seats you'd like to add.
Click Add seats.