I assume you mean adding to a breeding colony...
If it's a female you are adding, add a young female, you can pretty much add in a female whenever. Occasionally, it may cause a fight from a over-protective momma, but most of the time, adding a female is no problem. If you want to add an older/breeder aged female, wait until the babies are 2 weeks old, or no babies at all in the tub.
Males, same as older females, just be sure there are no pinks in the tub. 9 times out of 10 there will be no problem, but it's just safer to wait...

If you are pulling the females out to give birth, it doesn't really matter, just add em in, and watch for fighting. Doesn't happen too often, but every once and a while, you may get a little cat fight..
Of course the best way, is to just hold back your females, and occasionally change out males...