Is it possible to find out how one was nominated for Sigma Xi and is this organization worth joining? I recently received an email from Sigma Xi saying the following:
"In recognition of your contributions to the advancement of science, it is our pleasure to nominate you for Associate Membership in Sigma Xi, The Scientific Research Honor Society."

From what I can tell, it probably isn't worth joining unless I am planning on applying to their grants. However, I'm a graduate student and would appreciate other people's opinions on the benefits of joining this organization. For example, is it likely I would be allowed to join later if I wish or is this something one is typically only nominated for once?

I am also a bit confused and curious about how they got this information/ what "contributions" they are referring to. No one in my lab or my program cohort got this email and I started graduate school fairly recently so anyone from my alma mater probably would have supplied them with a different email address not my email address at my new university. I did an oral presentation this year and am on a paper that has been accepted (not a primary author though). I guess I'm just very curious since no one I know got this email?
The likeliest answer is that one or more of your professors nominated you. I don't think they nominate a lot of people, so you are highly thought of by some, at least. I've been a member since approximately forever. It is worth a line in your CV, at least. 

But if you are at a large university and interested, generally, in scholarly pursuits, it is worth joining as some chapters have periodic meetings at which you can connect with other people in other areas. This can be interesting and stimulating, much like Sherry Hour after Dinner at British universities. But it can also lead to collaboration. Many Nobel laureates are members.  

I don't see any reason not to accept the invitation.