The invite thing has a simple reason: There are two types of for a guild..
There are those people who want become membersand come asking the guild. For them is the court and signing thing.
But there are also people who fit the guild so perfectly, the guild wants them. For those is the invite.
As an example an magic guild: There are those younger sons of nobles, peons, etc, who want to be magickers, live good and wealthy. They come to the official guild court and were tested and perhaps teached.
But there are also those talented, who your mages see on their travels/jobs, and had never thought about your guild,cause they have no probs. Your guild wants them, cause they are your real human resources. So the finder invites them, where he founds them.