At the end of Chapter 4 of the Main Quest, My Guild functionality will be added. From here, you can receive Guild#Rank Rewards, also, you can do mundane and trivial tasks such as decorating your room with furniture you own; you can acquire furniture through the Friend Gacha, event item exchange shops (such as the ones during major events), or reaching a certain Guild#Rank.

As usual, more details will be added when announced.

Rank

Under Options > the top left button, you can view your current Guild Rank and how much Guild XP you have; you gain Guild XP by increasing the levels of your units.

Rank Rewards

So far, Rank Rewards appear to be accessible under the Shop through the 3rd button. Details as for what it offers is currently unknown.