Sometimes you're the one to capture a tile, and it really needed to be a guildmate such as if your demo happened to be the one to down an enemy fort but you don't have a free fort. You could just abandon the tile and let your teammate take it at their leisure, but then you run the risk of your enemies seizing the tile before the handoff is complete.
There are two options for handing off the tile. One requires work from your guild leader and one can be accomplished in their absence.
Leaving the guild[]
If you leave the guild (or are thrown out), your guildmates can then attack your tile. Any backup that is inbound but has not landed yet when you leave the guild becomes an attack, so your guildmate can send backup and then you leave the guild (or are removed from the guild) immediately before it lands. Their backup becomes an assault on your NPCs, as well as any other backup that was already on the tile, so it is important to send them home or coordinate with the people with backup so they can recall their backup before the handoff.
This is a dangerous maneuver as there will be a period of time when the tile has no backup; if the enemy attacks at that point they will only need to defeat the NPCs to capture the tile.
Since it's an attack, this can only be done outside of the NCP; during NCP players cannot attack other player's tiles.
After the tile is taken from you, it is safe to rejoin your guild.
Abandon the tile[]
If you select to abandon the tile, the process takes 6 hours (may vary by world). You can use the time remaining on the abandon and the game clock to determine the exact time the abandon will be complete. Your guildmate schedules backup to land a few seconds after your abandon completes.
When your abandonment of the tile completes, all backup is sent home. Your guildmate is attacking an NPC tile so this can be done at any time and NCP is not a concern. Since your backup is in place up until the second the tile is abandoned, this method is safer but does require the six hour waiting period.