The size and staffing of adventurer guild chapters varies wildly based on the location and the dangers inherent in the area. So a small city that is ringed by larger (better equipped to deal with danger) cities may have either a small guild with 1-2 leaders or organizers, or they may not have one at all.
For those towns that do not have a formal guild, often times they will use the bulletin board in order to help get the word out about dangers in the area. So often times you will see a messenger from the guild moving about and posting bulletins before moving onward. They usually act as impromptu recruiters/pitchmen for the guild as well and are happy to answer questions.
All active guilds have memberships available. Occasionally an area will be “overrun” by adventurers. When this occurs they look for applicants or members willing to relocate and they provide travel expenses. This allows them to get people to the area they are needed most.
The application process is simple. A form is filled out and the adventurer is given a magically imbued card with their information on it. They will test applicants with a supervised outing to determine skill level, and then they will make sure to send them on appropriate contracts. If one adventurer cannot handle a contract themselves, the guild leaders are allowed to pair people into groups in order to make sure the contracts are successful.