I have a strong interest in XYZ. Im a senior and I wanted to join XYZ this spring semester but I can't due to some personal circumstances. I still would like to join and is thinking about joining a grad chapter. My question is what is the difference between joining undergrad vs grad?(if this question can be answered)
The major difference is the experience. Joining in undergrad forever ties you to that college and that chapter, whereas joining a grad chapter ties you to that chapter only. In undergrad, you get to experience Greek parties (as a Greek, which is different than from as a non-Greek), plot days (if your school has plots), undergraduate programs with prospects (always fun hehe), having Neos, etc. In graduate chapters, the process is usually conducted by people that are much more mature and at a different place emotionally than undergrad. That may or may not be a good thing depending on what you're looking for.
In short, if you love the organization and you want to be XYZ until you die no matter what, then grad is still a great opportunity. If wearing letters on the yard, party-walking, going to step shows, etc. is really important, then maybe you should still try for undergrad. Remember, it takes a while to do grad as well.