Master invitation

The Master invitation is obtained within a Clan Citadel by a clanmate, who's rank has the permission to begin a private meeting, by attempting to enter one of the private meeting rooms upstairs in a tier-2 or higher keep.

The master invitation is then active for 15 minutes, in which the person with it can use it on other clanmates to invite them to the private meeting, and enter the meeting room. The clanmate must have Accept Aid set to on, and will receive the Clan meeting invitation if they accept. Once the time has expired, the meeting host can no longer invite clanmates to the meeting, nor enter the meeting.

Transcript
Citadel Meeting Room You have organised a private meeting in your clan's citadel.

Use this invitation on other players to invite them.

The invitation will expire in 15 minutes, or if the citadel is re-created.