When adding new team memebers... off set the ramp up time by

1) having pre built machines ready and and easy method of getting the lastest tools, code base etc.
I'm fortunate enough to be at a client that has a machine ready built and loaded when the dev arrives, all they have to do is grab the code.

2) have tasks broken down so that dependencies are as minimal as possible. In other words, to over come the mythical man month issue (as recently mentioned on slashdot) make sure the tasks you hand out have few dependencies on each other. That way the new dev is able to be productive fairly quickly.

Here's our historical lead time... the bump in Jan is due to added work, by 2/18 we had added 4 new people over the last two weeks. And amazing the time starts coming down:

Here's our averag work time:

again time ramps up as we are adding more tasks, but then starts inching back down through out Feb and March.

It's not that we beat the Mythical Man Month, and in fact I still believe the book and idea are highly relevant. But if you can break the tasks down and reduce the dependencies between the task then you can mitigate the effect.

The tool used in this case is from AgileZen.com and some of the wild swings are due to inexperience with the system initially... but our average times as measured by the tool are matching real life. Also the tool appearst to measure in 24 hour days and 7 day weeks. so it isn't as bad as it looks. :-)