Are you looking to integrate a new company into your existing Azure AD tenant? Or perhaps you need to share your tenancy and Office 365 services with more than one company? If you find yourself in a position where you need to sync users from another domain and have already configured AD Connect, then there is a way to add the second domain to your current Azure tenancy, so you can sync those users from the second domain.
Before proceeding, make sure you review the supported topology when it comes to AD Connect and multiple domains/tenants. You can find the information in this Microsoft article: https://docs.microsoft.com/en-us/azure/active-directory/hybrid/plan-connect-topologies
However, there is one limitation that will most likely cause you the most grief, and is probably why you landed here in the first place. You can only have one AD Connect instance per tenant. So, if you need to sync users from a second domain, you will have to follow the process outlined below.
Establish Domain Trust Before proceeding, you need to establish an AD trust between the two domains. If you have not done this yet, you will need to perform this step first. If you are not sure how to do this, you can refer to this guide: https://www.azure365pro.com/how-to-create-two-way-transitive-trust-windows-server-2008-r2/
OK, now that we have our domain trust established, let’s step into the process.