Before I begin saying anything, let me say that yes, I've put some gold and EXP into the guild, but I have a few questions that need to be answered. Keep in mind, this game still has some work to do so these things may not be final:
1: How do you gain more members?
2: What are the perks for each level/rank up?
3: Is there a way to make your guild mark (or will there be a way) appear on any equipment you have?
4: What is the current or planed max for a guild's level/rank?
5: What do guild quests do?
I'm sure there's a ton of other questions from others, but these are the ones I'm looking for answers to for the moment.
1. Raising your Guild Grade - +5 for each level and 40 total at Grade 4.
2. You get a new Guild Hall at Guild Level 5 and Level 10, Guild Grade raises max members, Rank is currently unavailable.
3. No system like that exists however it's always possible. You are able to change you guild Emblem which is displayed on the guild menu and rankings, but it's currently not available.
4. 10, but only 5 is currently possible.
5. Rewards tokens you can give to the Donation NPC for EXP and Gold contribution.