Guilds gain more member slots as it levels up - 2 per level. When you can start doing weekly quests and contributing to the guild it will level up pretty quickly, then you can start transferring characters in one by one. The maximum guild size is 100 members if 3 guilds fits all of your members then 1 guild at max level will fit them all.
Oh I’m so glad it levels! I’m an alt-aholic, and have a guild I plan on only having my family in, but just my alts alone are taking up most of the slots. It’s good to know it gets bigger as we go. Thanks for the info!