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:

1. 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.

2. 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.

3. 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!

Great answer from Jeff L. Related to idea #1, it can help to look at postdoc A.

If Postdoc A is comfortable doing this, she can try some interruption deflection techniques. Here's an example. Suppose she got interrupted while she was trying to say, "What we found most interesting about these results --". The trick is to repeat just the beginning part several times in a row until the others stop talking: "What we found most -- what we found most -- what we found most interesting about these results was etc." You don't have to raise your volume a lot for this to work, but it does help to use a slightly higher pitch than normal, remain very calm, and kind of extend that last word "most" a little extra.

A person who interrupts is successful with his interruption if someone else responds to what he says. What I'm getting from your question is that Dr. Prof and Postdoc B provide these responses to each other. That could be hard for a bystander like you to interfere with.

I wonder, since you work closely with Postdoc A, if you and she could make some co-presentations. It would take careful preparation, agreeing ahead of time on your content and your outline, and who's going to present what.

I have no way of knowing if things have reached the point where you'd like to ask the university for help, but if they have, it might be worth talking to your Title IX coordinator about what's going on.

I do not know about the culture of your research environment, but in my experience, the purpose of group meetings is discussion. Presentations get interrupted frequently, and the advantages and disadvantages of particular research are discussed bluntly and openly within the group. If A is interrupted and B is not, then that may mean B is boring.

It may be that some members of your group are interrupting inappropriately, possibly for sexist reasons. But don't assume interruptions are necessarily bad.