Cool idea/thread.
Let's see...
Apprentice: the Grunts. They do the simplest day-to-day stuff for the guild. Act as couriers/messengers. Run "errands." Perhaps some reconnoitering and "investigation" of missions considered "minor" by the guild at large. Only permitted IN the city or its immediate (less than a day away) surroundings. Most likely, they answer/report to one of the Journeymen and/or mid-level clerks who gives them their assignments.
Jounreyman: These guildmembers have advanced enough that they could be sent out on missions away from the city (caravan guards, rumors of treasures the guild would like to gt their hands on in the neighboring country, etc)...and/or "high profile" missions within it (the Countess/Merchant's Guild/City Council needs someone to...). Also, as stated above, some/certain Jounreymen would be included in the guild's administration, overseeing/assigning missions to their peers and lessers. Magely/scribe types who maintain records/archives/dues for a particular section of the guild, etc.
Senior Journeyman: Advanced veterans above the normal "Journeyman" status. Mostly likely have traveled fairly extensively. They are sent further afield as necessary or as they desire. They have a modicum of autonomy to "seek out" missions on their own (which, obviously, should be reported/logged with the guild but do not, necessarily require guild "approval" to undertake). Those no longer wishing to lead a life "on the road" could/would serve as high-level administration and "trainers" of various adventuring skills for the lesser members. The "Master Clerk/Bookkeeper" whom the lesser clerks/bookeepers report to would be a Senior Jounreyman. They answer only to the "Masters" and are assigned to missions of extreme importance/urgency or requiring the utmost "discretion"...anywhere in the world the guild is able to reach (including things like the Underdark, other planes, seafaring to other continents, etc.)
Master: I see this tier as a very limited number of members. Maybe even just the original party who started up the guild. I'd make 6 to 8 seasoned adventurers of various classes. They each have their own agendas as well as a casting vote to any serious matters the guild puts to a vote. Being awarded an "honorary Master" title is extremely rare and difficult, reserved for only the most successful and unfailingly trustworthy guildmembers. One of the Masters is the direct superior of the "Master" [Senior Jounreyman] Bookkeeper, keeping a close eye on dues brought in, percentages of treasures found, taking his/her pick of the best items for himself (and/or his fellow Masters and the Grandmaster). This Master would also keep an eye out for individuals of particular or peculiar talent (whether or not he/she would be interested in assisting such individuals or stifling them would depend on the kind of person you make them).
Grandmaster: A single individual, the leader of the original party who set up the guild. Most likely the highest level guildmember (though the other Masters are probably within a level or two of him/her). He/she likes to be kept abreast of the guild's activities, but generally is not personally concerned with anything below the Senior Journeyman missions (rather, nothing below that is brought to his/her attention...the assumption is the other Masters and Senior Journeymen are more than capable of dealing with those lower subordinates). Most likely an extremely charismatic leader with strong ties and allies in the top tiers of the city government/nobility for their numerous exploits/"saving the days" in their younger adventuring years.
That's all I have off the top of my head...and two cups of coffee.
Again, great idea.
Have fun and happy guild-building.
--Steel Dragons