In the current build of the game a Guild is the only way to formally create a group with other players, guilds also have a limited number of slots for players to join ~20, which requires you to create multiple guilds if you wish to have more members.
Starting a Guild
Starting a Guild requires a Guildhall and offers a few benefits. Players, who don't have the right skills to build a Guildhall, can create a Improvised Guild ("Party") with up to 5 members.
Using tinkering, one can create a Poor Guild Lock which can be placed on buildings and allow those buildings to be accessed only by members of the appropriate Guild. Also, regarding Arrow Watchtowers. Normally they attack any criminal(red) player around them automatically, as well as hostile monsters. But, a Guild member near friendly built towers will not be automatically attacked by said towers, thus allowing your guild's defenders, thieves, etc. to have a safe haven within your town even while tagged as a criminal.
As an additional benefit, normally when you would damage another players building you would become a criminal for vandalism, while in a guild if you attack a building of another player in your own guild you will not become a criminal, this can be helpful if a friend needs help removing a building or wall.
Guildhalls create Guild Territory around them. Walls, Gates and Watchtowers can extend this Territory. Livestock counts as a Guild Property, if it is in the Guild Territory. Enemys cannot spawn in the Guild Territory, also attacking a player on your own guild land will not give a crime ticket or turn you yellow.
Currently the ranking system within a guild is limited. There is only a distinction between a Guild Leader and regular members. Future updates to the Guild system are expected to give additional functionality to the ranking structures.
When a guild leader quits the guild, the guild's leadership falls to another player in the guild, usually the leadership position will go to which ever character joined the guild after it's leader, in descending order. This is how it worked before the recent 04/2016 update, unknow if that it still works this way.
NOTE: info stolen by forum post: http://www.treeoflifeforum.net/board/viewtopic.php?f=2&t=2998
TODO: need a check
Visit this page for a list of Known Guilds.