I know this may be common sense but alot of people for some reason don't know about this and i dont want to be a leecher on this site so im trying to contribute some how.
1. type "Hi, wanna come XX ?" where XX is the name of the instance or raid and select it and press CTRL + C to copy it.
2. now clear what you typed and type "/who x - y" where x is the minimum level you want in the group and y is the maximum level, e.g. "/who 32 - 37" is what i'd type for sm lib
3. Now the who list should pop up, simply right click the first person on that list select whisper and press ctrl + v to paste your message to him. Repeat this for everyone on the list, you should be bombared with message saying either inv, or no ty.
Another thing you could type is "/who z x - y" where z is class you want to join e.g. "/who priest 33 - 36".