I believe what you are saying is that when you select all focus points to be used in a group portrait and when you partially depress the shutter that only one or two points will light up as achieving focus. Dependent on the mode you are shooting in, this would be normal focusing behavior.
I have never used the A-Dep setting (I perfer to think more than the camera does) but is my understanding that the A-Dep mode will "look" at all parts of the scene under the focus points and choose the appropriate aperture to have everything or everyone under the focus points in focus. This of course in lower light or any real depth in the posing of the subjects may well lead to the use of an aperture than you would prefer not to use for a number of reasons.
This leads to your other question about using a wide/fast aperture like f/2.8 to capture a group portrait, it is possible but to do so all of the subjects must fall within the limited dof that is available at this f/stop or any other aperture chosen for the composition. This may mean that all of the subjects be posed all but perfectly parallel to each other, in a group of more than three this may not be possible nor even the most attractive pose. The suggestion to use a dof calculator is a very good one.
Knowing or understanding the dof available for the particular body, lens, subject distance and aperture chosen is a vital first step to achieve the particular look of an image the photographer is trying to capture. From that point knowing then the dof one can concern themselves with the posing and the specific exposure requirements required to capture the image theyt envision.