Guilds level through guild missions and by having characters in your guild level up. Level 1 guilds cap at 30 members, and the max cap is 50. This limit increases by 5 per level, so a lvl 5 guild is capped out on members. You still gain additional benefits from leveling a guild past 5. Lvl 3 is when you can declare war on other guilds (and be declared upon) up to three times a day. The first war dec is 100g, and gets into the thousands by the third war.