Being Ostracized in Research Group I currently am a sophomore in a US college. I have been accepted into a semester-long undergraduate research group. Officially, we have a faculty advisor as well as graduate student mentors. In practice, we mostly only meet with our graduate student mentors.

The problem is that, from the first day, one of my teammates deliberately ignores me in every group meeting. I do not know why, but they talked to all other teammates except me. Every time I tried to interact with or discuss with them, they skipped me and pretended to that they did not hear what I said. 

(I said 'deliberately ignores' because there were occasions when only I and the person in question were present, and the person still refused to communicate with me. However, the person engagingly talk to everyone else. )

My other teammates are not talkative at all. In a meeting, they are the only person who casually led the conversation and left me out of the conversation. I feel ostracized and more and more mentally bullied.

The situation makes me feel extremely unhappy right now. I am quite certain I did not do anything inappropriate to them because the issue arose from the first time we met. For now, I feel out of place and think that I might not belong to this place.

What can I do for now, besides quitting?

One option I am thinking is to approach graduate student mentors. However, I am holding back from doing so because it seems that I am the only person who has this problem. Also, since our mentors are also present during the meeting, I have a feeling that they already knew the problem exists.

For clarification, I had an issue with only one teammate and not the others, and the issue is with that person refusing to communicate with me, even when only two of us were present.
Based on the updates, there are a few things I can offer in terms of advice. The first is that given it's a short amount of time remaining on your term, stick it out. Especially if this is going to impact your college credits/marks/etc. Sure, it's a crappy situation but try to make the best out of it as not to impact your marks or the progress of your degree.

Once completed, don't return to the research group. Take the opportunity now to find research groups within your institution that are nicer and more willing to host a student because it sounds like the current group sees students as a hindrance rather than as a collaborator or colleague.

There are avenues you can take to make complaints about bullying in the workplace, such as speaking to professors (not unless they're the one involved in bullying) or going to a human resources division. However, this may not be in your best interests to do anything 'formal' at this stage of your career given you aren't committed to this lab group on a fixed term contract as an official job. I would be talking to the student mentor you mentioned in the comment about your experience.