Have him take the Leadership feat, and his followers and cohort represent the core memebership of his guild (while it's small, most or allof the folks in it shoudl be pretty loyal tohim; if he's charismatic enough to have a LOT of followers, then it's fair to say he's enough of a leader to keep that level of personal loyalty up even with a not-so-small Guild.
Eventually, though, if he's in a big enough area (and he'd BETTER be, if he's got more than a dozen followers), he'll have guild members who are not also his FOLLOWERS. Some of whom may come to rival him directly in terms of power, and sway over the membership (read: their own followers). Which means you can, as a GM - at your discretion - introduce a power struggle to take the guild away from him (maybe it succeeds, maybe it fails, maybe the guild falls apart and becomes a half-dozen or more smaller, warring guilds all fighting over the same turf).
Regardless, just taking LEadership should give him the MEMBERS.
As for OTHER considerations - the members would come with standard NPC gear for their level, which means, most of them won't be able to afford much, if anything, in the way of equipment.
Certainly not a place to eat, sleep, train, etc.
For that side of things, I suggest you or s/he go out and buy the Stronghold Builder's Guide. The thief player can sink their own money into building a stronghold (read: Guildhall). Or, he can take the feat "Landlord", from the SHBG, which provides seed money (by character level) and "matching funds", towards the construction nd equippage of a stronghold.
The new guildmaster will obviously want to build somethign concealed, so all the tricks for keepign secrets secret will come into play, and certain sorts of construction won't even be possible (so, probably no floating citadels hovering over the city, neon signs that read "OFFICIAL THIEVE'S GUILD. NO ENTRY EXCEPT BY APPOINTMENT!" ... unless your game is set in/on Discworld, heh).
The SHBG actually LISTS a training area for Rogues as one of the sorts of rooms you can set up, btw. ^_^ So your budding Godfather can even set up a thieve's academy of sorts, training youngsters in the fine art of "active wealth redistribution". ^_^
Me, I'd le the Guild set up small "strongholds" at various locations around the city. A safehouse HERE, a Tavern with a secret room or two, the backroom of a warehouse over THERE, and so on. It might be easiest to just state a GP value for the Guild's citywide "holdings", and leave it at that.
MEanwhile, as for how much money is made ... count each criminal activity as one "income source" by the SHBG rules - possibly by neighborhood for somethings, possibly city-wide for others (whichever makes sense for a given activity; information brokering woudl be IMO city-wide, but pickpocketing woudl be per neighborhood and per market place of significant size - e.g., pickpocketing operations in each of the Grand Market (better economic area), the Tavern-and-Brothel District, and the Low Market (lower economic area) might each be a seperate income source. Managing prostitution would primarily be centered in that same Tavern-and-Brothel district, but the rest of the city might be able to support enough "incidental" prostitution to count as a second income source. Outright cat-burglar robberies wold probably be based on operations in both the Middle and Upper class districts (meaning, two income sources, since that sort of activity has to be kept infrequent and/or small enough not to draw too much attention on the Guild).
There're also protection rackets (by each business district and maybe each market), general smuggling (city-wide), an illegal slave-trade (one per city), possibly a narcotics trade (one per economic strata, I'd say - what the poor will buy for drugs, and what the rich will buy, are often fairly different), and so on.
Of course, in any city of significant size, most or all of the available sources of income may already be staked out - so the new Guild will either have to start by going to war to SIEZE one or more of those sources, or, they'll have to get creative and think of something NEW.
I'd figure that represents the dues / tithes / larceny liscenses / "please don't hurt me (again)" money / etc, paid by the lesser thieves to the Guild proper.
The trick is, that money goes first to MAINTAINING the Guild. LEgal fees (if the locals bother with trials), graft and bribes, patching the roof of that tavern withthe secret rooms, and so on ... that all costs money. And then there's the simple concept of expanding the guild. MAYBE even into entirely new cities! (why stop at just ONE, eh? ^_^)
Little or nothing should truly "trickle upward" into the PC's pockets. However, they have (in return for their investment of two feats and a LOT of real-life time and effort) a possibly huge array of resources. Any thiefly equipment or possibly-purchasable item (legal or illegal alike), the character should be able to get their hands on it in relatively short order - and may or may not have to PAY for it (hey, he IS a THIEF, after all). Anything they can't get directly, well ... if it's in the Guild's controlled/influenced area at all, they should be able to figure out WHERE it is. The should also have access to a wide range of skilled underlings, and even skilled craftsmen, through their contacts within the Guild.
