1) If your signup to a raid doesn't mean automatically you are the part of the raid. The raidleader has to Confirm your signup this will happen in at least 1 day before the raid. if you see yourself in "Confirmed" and as a part of a Signup Group, you can be sure you will be invited when the raid starts.
2) Please do not late. If you signed up a confirmed for a raid, please be online at the raid instance's entrance in 15 minutes before the raid starts. And please have all neccessary items (potions, various armorsets if needed) right with you in your inventory.