Any characters with the guild artisan background are assumed to have free membership in one Waterdeep guild.
To join a guild, one must possess a background, proficiency, or a status that the guild values. For example, a character with the sailor background is welcome to join the Master Mariners’ Guild.
Guilds charge members monthly or yearly dues that vary. For simplicity, you can have a guild charge 1 gp per month or 10 gp for a year’s dues paid in advance. A guild can expel a member for any number of reasons, not the least of which is failing to pay dues.