TeamPlayer is an addon to provide a set of assist macros. The first part is used to send chat messages requesting that people assist or defend you, the second part is used to assist or defend those requesting it:

Macros available
/script TeamPlayer.CallAssist() - This will send a "ASSIST me on - [mob name here]" to the correct channel
/script TeamPlayer.CallDefend() - This will send a "DEFEND me from - [mob name here]" to the correct channel
/script TeamPlayer.Assist() - Assists the last person to call for assistance
/script TeamPlayer.Defend() - Assists the last person to call for defending

The addon monitors incoming assist and defend requests and stores the last person to ask for assistance or defending. This means you don't have to keep editing a macro for your assist button. You create a single assist button (and a defend button if that's your style), and it should work no matter who's calling for assists.

A default channel can be set by calling '/tp channel [full channel name]' or can be passed in the macros by passing it as an argument to CallAssist() or CallDefend() like CallAssist("partysay") but if no default is set and no argument is passed the addon automatically selects the appropriate channel in this order:

If you're in a scenario, /scenariosay
If you're in a warband, /warbandsay
If you're in a party, /partysay
If you're in a guild, /guildsay *
If none of the above, /say *

• - Probably not all that useful

Update History
v0.92 - Fixed default channel and channel as argument to macro bugs. Thanks to Fycus for help testing and fixing. With this, I think everything is working, will likely have a v1.0 release later today.
v0.91 - Fixed 2 problems, 1 with default channel selection, the other with initialization.
v0.9 - Initial beta release.

Enhancement Ideas
I'll probably be adding a macro to target someone who asks for defending so healers can use that.
I also think it would be interesting to create a small UI window to keep track of who's calling for assists, maybe show assist calls in a way that stand out more in the UI.

I welcome any other ideas for improvements or enhancements.