I am definitely going to do the second part of your response. With regard to the first part of your reply, I have volunteered at other organizations as well, but this is the first year that I have volunteered with organizations of the same philanthropy cause as the sorority.

And if you still don't think that will work, what advice do you have with regard to volunteering?

Signing up to volunteer for something just as a gateway to the sorority is probably a really quick way to get a creepy stalker rep. If it's something you would have gotten involved with anyway then that's fine, but be prepared to be asked about it and have a sincere answer. If you feel like you need to step up your game as far as extracurricular activities and volunteering, get involved with things that mean something to YOU -even if they have zero to do with either sorority.

^ That's what I was trying to convey

If these locals are anything like NPC groups, then volunteering and your commitment to a particular philanthropy is going to be only one part of the membership selection process.

Above almost everything else, NPC groups (and I assume these locals, as well) look for a well-rounded personality that gets along really well with the women in the chapter. Most social sororities participate in a variety of activities: social, philanthropic, academic, personal development, etc.

OP- you sound like you're putting all your eggs in the service basket. Are these groups PRIMARILY service groups? You also sound like you're overthinking the application and reasons for joining. Practice your conversation, chat with people you know who are in these groups, and read through the pages and pages of advice here.