With The War Within Pre-Patch Guilds are now cross-realm, allowing players to invite any players from their region to their guilds.
This is another step on Blizzard's journey to improving the game group accessibility, which started in Shadowlands. Cross-Faction raiding was allowed in Shadowlands Patch 9.2.5 and then Cross-Faction guilds became a feature in Dragonflight Patch 10.1.
This next step continues to break down barriers for players, allowing them to create cross-server communities to raid that aren't content-restricted for greater flexibility when choosing servers, factions, and guilds.