First, be sure to email the group and/or the group facilitator. Let them know what’s going on so the group doesn’t think you poofed into thin air. Maybe it’s not a good fit, maybe you wanted to submit more (or less) frequently. Whatever the reason, be sure to communicate it.
To leave, go to your group’s page. Click the settings wheel and select Leave Group. This will end your membership in the group. Your submissions and critiques from the group will remain visible to you on your Dashboard. Your discussion comments and critique comments given while in the group will remain visible to the group’s members.