there's a mod called guild log i use. it's pretty old, and i can't find it anywhere anymore, but thankfully is still works great. every time i log in (or toggle it from guild pane) i see a log that shows everyone who joined guild, left guild, leveled, changed ranks, or had their note change. it also saves a history until it's cleared. best part about this, it catches things like transfers and name changes that the normal guild log misses. it wont notify you right away, but sooner or later you will see the log, which is good enough for me =D if you want to PM me with an email address ill be happy to send you the files.
EDIT: forgot there's this
http://www.wowinterface.com/download...uildCheck.html but as i recall i liked guild log better