To remove yourself from a group, just follow these steps:
If you're having difficulty accessing your group online, first clear your browser's cache and verify you're signed into the group with the correct email address (ie that you're not trying to access an HSU group using a personal Google account or vice versa), then try again. If that doesn't resolve the problem, check that the group is still active and hasn't changed its address - or, if you're new to the group, that you accepted the original invitation. Don't bookmark groups - you should always access groups via the Google Groups home page.
If you're unable to post any messages to a particular group, try clearing your browser's cache. If that's not the problem, visit the group's home page; this will tell you whether you have the right email address for the group (if you're not recognized as a member, you have the wrong email address) and whether you're allowed to post to the group (if it's announcement-only, then you can't post). If you're having this problem with a group you manage, your ability to post may have been overridden by another user, in which case you'll need to edit the posting permissions next to your email address.
You should set up a separate personal Google account for your personal groups. That way, the two groups won't get mixed up and you'll be able to keep things a lot more organized. Plus, the HSU implementation of Google Apps is a little different than for personal accounts, so you'll have better control all around if you keep the two separate.
Click on the Group settings link on your group's page, then click on Access. Here are some guidelines for managing your group settings:
Log into your HSU email account and go to the Google Groups homepage. Click on the "Create a group" button in the upper right corner of the "My groups" box.
Creating a group is a two-step process. First, you name your group, create an email address for it, set the types of access to be allowed (Public, Team, Announcement-only, Restricted), and enter a description of the group. Once that's done, you can start inviting people by entering their email addresses. That's really all there is to it.