LPT: If someone takes the time to invite you to something, immediately let them know "yes", "no", or "I'm not sure, but I'll let you know as soon as I can"

Other people are using their time and resources to plan something. They cared enough about you to reach out to you and invite you. Their time, planning, and organization, is often dependent on YOU getting back to them promptly, one way or another. Have the consideration to let them know as soon as you can whether you're in or not. Not responding as a way of saying "no" is just plain rude, and you're making other people wait around for you when they could be doing something else. If you're not sure if you're available or not, it's okay to say that's you're not sure and that you'll get back to them, or to tell them that they can plan around you, and you'll join if you can. Also, keep in mind that if you continue exhibiting poor etiquette in responding to invitations, it will mean that you won't get invited to things as much, because you'll be perceived a flaky jerk with bad communication skills who inviting is more trouble that it's worth.