This was a feature request I gave to Ethan a long while ago to propose on the D-List, I'm not sure how far it went but I would like to raise it again.
Basically, the idea is to allow for high ranking members of a secret society to invite other nobles to join without having them travel to the guildhouse. It would work exactly the same way a priest can convert a noble with a private sermon, except with the obvious alterations. This would be a major change to secret societies especially when the two nobles are in enemy realms. What excuse could they possibly have to get to their enemy's capital, but if the two were to know where their armies were going to clash it would be that much simpler to send an invitation.
Here is the actual Feature Request, you may have already seen it a year or more ago...
Feature Request: Invitation to Secret Society Without Nearby Gathering Place
Summary: Remove the joining restriction on secret societies.
Detail: As a priest can convert nobles through private sermons, so to should a senior member invite a noble into their secret society. This would be done without the need of a nearby gathering place, but the two must be in the same region. The gathering place would still be required for the amount of nobles the society is allowed to have and for treasury purposes.
Why:
If a noble is invited to join a secret society, but the only gathering place is in the enemy realm’s capital, it is impossible to justify a reason for that noble to travel to the capital. Allowing a senior member of the society the ability to invite nobles without a nearby gathering place removes a crucial obstacle in spreading the society. While guilds function through public buildings, secret societies should function privately in this way.
Easy to implement?
