Hi,
Each Mac needs to be on the Same Subnet of the LAN.
In iChat or Messages go to the Preferences > Account and enable the Bonjour account.
In Messages also do:-
In Preferences > General Section Untick the item about linking the Buddy Lists.
In iChat the Buddy list should appear on the "desktop"
In Messages go to the Window Menu use the CMD + # keystrokes for the Bonjour account to make it show on the "desktop"
I have 10 (other then the iMessages account) accounts logged in and my Window Menu looks like this.
My Bonjour account is so far down the list that there are not CMD + single numbers left.
Selecting it in the Menu list will also bring it to the Front.
Whilst not being the same shape it should look like this is you are the only one logged in.
I have three computers and launching iChat on another creates a Buddy list the List (I will come on to the naming format).
As I am "Ralph Johns" in the Address Cards on each Mac, I have moused over this one to show it is Ralph Johns on my MacBook Pro.
You can't make Buddies join the list.
You apppear to others as the name in the My Card in your Address Book/Contacts App.
They appear to you as the name they have in the My Card on their Computer.
Using Screen Sharing I grabbed a Pic from the MacBook Pro and then sent it to the iMac in a Bonjour TeXt Chat
Above it in this pic is my Address Card.
You can see the Buddy Pic is the MacBook Pro one
You can see the Ralph Johns-iMac is the Buddy Name the iMac Bonjour account shows the MacBook Pro
There can be issues if the computers have the same Name.
See System Preferences > Sharing.
This can happen with DiskImage updates where an IT department generates an global form of the updates and uses a DiskImage to distribute.
In this case other computers may not appear in the Shares secion of the Finder's Side Bar.
There can obviously be issues if the My Card on each computer is the same due to either using a Global Address Book or the DiskImage format of updating.
Once you pass 255 other computers on the LAN you will have issues as well and another router will be needed and it will issue slightly different IP address (Different Subnet of the LAN).
9:25 pm Tuesday; April 1, 2014
iMac 2.5Ghz i5 2011 (Mavericks 10.9)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
Couple of iPhones and an iPad