Tell them to make inquiries to the D&D AL (Dungeons & Dragons Adventurer's League), formerly known as the RPGA (Role Playing Gamer's Association - they may have seen mention of it in promotional materials found alongside D&D 3rd Ed. manuals and other products like Dungeon Magazine, or Dragon Magazine before they stopped publication).
This is assuming that you mean D&D campaigns run online (VOIP, Discord, and/or Foundry), and not the D&D Online MMORPG, of course. Barring that, and taking your unknown geographical location into account, they may also be able to find organized/advertised gaming groups by making the rounds of all of your nearest brick and mortar gaming/comic shops. They're well known as consistent sources for tournament organization for things like Warhammer and Magic: The Gathering as official retailers of said products alongside the D&D content released by Wizards of the Coast.
Tell them that I said good luck as well.