I have been gaming since 1999, so I know I speak for more people than just myself. If a person pops-up a group invite (or a guild invite either for that matter) in my face in-game, right out of the blue, I always just click "NO" without even giving it a second thought. Some may not find this as rude, but I and many others do.
If I receive a tell from a person who is in the reasonably close geographical area to me and needs help, at least 90% of the time I will help them. Random invitations to group tell me nothing about where a player is or what they intend to do. Why would I join a group when I have no idea where the group is working, or what they are trying to do? The same goes for a guild invitation as well. Why would I ever just click "YES" and join a guild I know absolutely nothing about?
If you need help, send a simple tell..."I need help with a boss mob--can you help me?" I believe you will find the help you need, and maybe make a new friend in the process. Please do not resort to the Mrs. Doubtfire approach to finding group or guild members: "Oh, Sir, it was a run-by fruiting!"