Devo

Part 1) Create Join Guild Function
 
This is a small logic into it:
First of all create 5 player variables and name them like: guild1,guild2,guild3,guild4 and playerguild variable.
And Then create an event that creates the command "/joinguild" and when the user writes this command it should show them the options box.
You can add up to 4 options there so you write like: Guild 1,Guild 2,Guild 3,Guild 4 and once the user selects one of these based on his choice you update the "playerguild" variable to the guild he choose.
 
Part 2) Create Leave Guild Function
 
Create the "/leaveguild" command that simply empties the "playerguild" variable.
 
Part 3) Guild Chat
 
Create "/guildchat" command that shows the text that player writes in messagebox. (global chat)
Create an event with conditional branch that checks "playerguild" variable to see what guild player is on and then show the message sent via "/guildchat" to players with same "playerguild" id.
 
Part 4) Guild XP or Reputation call it whatever you like.
 
Create "guildxp" variable as global.
Create conditional branch to check the guild player is in using "playerguild" variable.
Then at player events,like kills,deaths and stuff add the code to increase and decrease the "guildxp" amount.
There should be a different guildxp variable per each guild of course like: guildxp1,guildxp2,guildxp3,guildxp4.
 
This is the only way i can think of with the source still not being released.
 
Part 5) The guild tag before players name.
 
Use the chat box commands to show the guild name before players message like "[ Guild 1 ] Hello world".
Example: \pv{playerguild} \param

It works fine for me: http://prntscr.com/pfcdtt (Tested on 6.123) 
P.s: Did you download full 6.123 or upgraded your old version to it ?

Because that's an attribute event you should run the event editor to do that one: http://prntscr.com/pfaoun

That's called a launcher it updates the player game files on every run if there is a new version released.