SharePoint 2019 / Project Server 2019. On Prim. Users in different domain than the servers.

Hello, I am a SharePoint newbie needing help. I was tasked to build Project Server 2019 for our PWO office. I was surprised that I had to install SharePoint then install Project Server. Our Server's are on #1 Domain and users are on #2 Domain.  I found some really great information on walking me through setting this up. However, I quickly realized that none of my users can see the Project Server Site I have built.  I realized that I had to add the users in SharePoint. However, It can not see any user accounts in Domain #2. I have scoured the web and I am at a loss. Any help would be really appreciated.

Thanks in advance.