You may have noticed some people spamming "X is a traitor!" or "I'm with X" or similar. These come from the ttt_radio command. You can use this too! You just have to bind the command to a key. If you don't know how to bind check out the Key Binding guide using ttt_radio as the command. However, after ttt_radio you need to add something else to tell it what you want to say. what those are are below.
yes="Yes."
no="No."
help="Help!"
imwith="I'm with ____."
see="I see ____."
suspect="____ acts suspicious"
traitor="____ is a traitor!"
innocent="____ is innocent."
check="Anyone still alive?"

Step 0: Make sure the console is enabled. To do this open options, and then click advanced and check if Enable developer console is check. If not check it.
Step 1: Open the Gmod Console. You may or may not need to join a server or start a single player world in any gamemode. When this is done press the Toggle developer console key, which defaults to ` (Tilda key).
Step 2: In the console type bind {key} where {key} is the key you wish to bind. For Example, if you want to bind the O key, you'd type bind o. This should give the output "o" is not bound if the key is unbound, if it is bound to something it will return "o" = "{command that is bound}"
Step 3: Now that you know your desired key is not bound, you can bind it with bind {key} "{command}". Where {command} is the command you with to bind. For Example, if you want to bind a key to say "I am alive." to o you would type bind o "say I am alive." You can look up other possible commands. You can bind any command you can type into the gmod console.
Step 4: Enjoy your new bind. At this point the key is bound and can be used as much as you want.
Step 5: If you want to unbind the key, you type unbind {key}. For example, if you want to unbind o you type unbind o and it will be unbound.
Unusual Keys Here are some keys that might not be as obvous, the Num Pad.
KP_NUMLOCK = Numpad Numlock
KP_SLASH = Numpad /
KP_STAR or KP_MULTIPLY = Numpad *
KP_PLUS = Numpad +
KP_MINUS = Numpad -
KP_HOME = Numpad 7
KP_END = Numpad 1
KP_UPARROW = Numpad 8
KP_DOWNARROW = Numpad 2
KP_LEFTARROW = Numpad 4
KP_RIGHTARROW = Numpad 6
KP_5 = Numpad 5
KP_EQUALS = Numpad = (Not on all numpads)
KP_INS = Numpad 0
KP_DEL = Numpad .
KP_PGUP = Numpad 9
KP_PGDN = Numpad 3
KP_ENTER = Numpad Enter