There are three types of methods that players can join guilds.
Open: Any player can join the guild as long as there are open slots.
Application: A player may apply and the guild leader will need to accept or decline.
Private: The guild will show up in the search, but will be marked as private and no one may join. This is best for players who have a guild with friends, who are content with their current members, or just don't want to deal with people trying to get in. This is a way to block newcomers.