For the record, my attempt at reviving the Mage's Guild was a whimsical idea I got one evening and posted pretty much right away. It simply represents a hastily written summary of how my PC might have gone about it, had he been resurrecting it; you shouldn't feel bound to those ideas in any way.
As long as the guild is about elitism, coolness and genius, I'm fine with basically anything you can think of. Invitation could be one way of making sure a mage has done something worthy of note before they can enter. Maybe invitation+test (quest) could work too. I suppose they could hire even lower-ranked mages if they feel someone is a genius in the making and will quickly climb the ranks. As the guild is highly competitive, members that don't live up to the expectations would likely be booted rather quickly. Conversely, being an Archmage is no guarantee of admission - just look at controversial RL scientists saluted as geniuses by some and ridiculed by others.
As for benefits, I think you may need to come up with something that nobody else has. Not a license to kill, but something almost as useful. Problem is, anything giving them more rights than the ordinary citizens would have to go through Milo, since it's not already established. Mostly though, someone's prestige as a mage would go up enormously upon joining the Guild. It is likely that he could charge much much more for his services with that pin on his robes.