You can build a 5,000-person database using existing business relationships by compiling contacts from various apps and services. This process involves exporting contacts from Google Contacts, LinkedIn, Outlook, QuickBooks Online, and other platforms into an Excel or Google Sheets file. The contacts can then be cleaned up, segmented, and imported into a CRM system, email automation software, or spreadsheet app. To make the process more efficient, you can use Zapier to automatically add new contacts as they come in.