How do I help minimize interruptions during group meetings as a student?
This sort of thing has roots way beyond academia.  There is a lot about the dynamics of women having difficulty being spoken over in meetings (though I imagine it could apply without respect to gender too, based on differing levels of assertiveness).  Maybe a few things to try:


When someone, even your advisor, talks over her, say "Wait, what were you about to say?" addressed to her, so it's not directly confrontational toward the interrupter.  This is as opposed to something like "Wait, let postdoc A finish" addressed to the interrupter, which is more confrontational.  That is, assuming you want to avoid a confrontation with your advisor.
Mention it casually to her after a meeting.  She might appreciate and/or be emboldened by the support.  Something like "What was it you were going to finish saying before soandso took over?"  This way you show you're both interested in what she had to say and also noticed that she was talked over in the meeting.  
Wait until the interrupter finishes, then bring the focus of the meeting back to postdoc A.  "Sorry, what were you about to say earlier?"  or "Hey, postdoc A, what was it you were saying about X a minute ago?"  This is probably the safest thing to try, and if you do it enough the others might even get the point!