Just drop us a line here in the form of a new thread if you'll be gone from in game or the guild for an extended period. If you don't let us know what's happening, we'll end up assuming that you've lost interest in the game and probably won't be coming back. Posting here will also help you retain the appropriate forum access.
If you need to leave the guild for a short time, just let us know here and re-inviting you will be a simple ordeal. Without a post here though, you might have to re-apply to the guild entirely.