Inside of the Guild Hall you can find two Guild Banners. One banner may be in your inventory at a time. To use a Guild Banner, you simply double tap/click and it will appear on the ground with a circle around it, this is the area of effect your Banner will have. Everyone, including non-guild members, are able to see your Banner. Non-guild members will not see the area of effect circle, however. Guild members who stand inside the circle will recieve that specific banners stat boosts for the duration of the banner. The Signal Banner is currently the only available banner.
There can only be one Guild Banner down at a time, and trying to set one up while another one is still running will replace the old one. Guild Banners are easy to use and are always available in the Guild Hall.