A Guild is a group of players within the same organization, run by players themselves. It could be democratic, anarchistic, totalitarian, etc. Any player can join a guild for whatever reasons, and more simply a guild can just be a group of friends. Each guild is supposed to provide mutual protection for all guild members, and a place where they can draw support and assistance from the other players (guildees, as they are often referred to).
To join a guild you need to go at that guild profile, and click on «Request to join this guild». Then your request must be accepted by a member of that guild with powers to accept new members.
Whatever your level, there is no limitation when applying for a guild. Some guilds, however, will not accept members below a certain threshold. You are unable to apply to join a guild if you are already in one.
The numbers in the column represents the last time a player accessed that server. example: an inactivity of 30 min they last accessed the game 30 minutes ago.