In informal settings, you might use あたし達, to your parents-in-law you might use 私達, and in business settings, you might use 我々 or 私共. It can depend on many things, including possibly your gender, but it doesn't generally depend on whether there is somebody of the other gender in the group you refer to.
The Japanese plural marker 達 as well as ら and others AFAIK, can work as associative plurals. This means that you can use 田中君達 to mean "Tanaka and others associated with him (whose names are not necessarily Tanaka)". Compare this to English "the Joneses" in which you would usually expect all of the people referred to to have the name "Jones".
Thus, あたし達 can be thought of as "I and others associated with me". Semantically, this isn't really different from "we", but it might give an explanation of why you don't need to change あたし to something else, just because there is a male in the group.
All this being said, I venture the statement that 達 and other Japanese plural markers can also be used an additive (normal) plural. For example, unless the speaker has a specific 部長 in mind, you would usually understand うちの会社の部長達 as the group of 部長s (and only 部長s) from the speaker's company.