Joining guilds is an amazing way to connect with more players and do bounties and get more rewards. 

The game will offer you to create or join a guild during the tutorials.

Within a guild you can set a bulding for discounted investment and have the cost of investment in celected building reduced by a certain % for all Guild members. Another building can be picked once in every 2 hours. 

You can also accomplish different bounties available in a guild and get special rewards. 

Guilds allow you to request for assistance in upgrading the Shop (accelerating timers).

When entering the Guild section you can see rating option: it allows you to see how you compare to other players in the guild, including net worth, town progress, prestige, bounties unlocked blueprints and help provided.