As an ISP OverSubscription is your friend. Typically ISP's will over subscribe somewhere between 5 and 20 to 1 for broadband cable/DSL access. What that means is that if you have 128k of bandwidth you should be able to sell somewhere between 5 and 20 times that much bandwidth on to people. How much you over subscribe depends on your customer base and your customer usage patterns. For example when I worked for an ISP in Alaska we could over subscribe nearly twice as much in the summer as we could in the winter (cause there's nothing to do in the winter but surf the net ... ).
One key part of managing this will be bandwidth restricting people, typically with a CIR/MIR type setup. As an example lets say you have a T1 to the internet (which is 1.544 megabits). You guarantee each customer 64kb no matter what but you allow them to burst up to 256kb if the BandWidth is available unused. In the case 64kb is the CIR and the 256kb is the MIR.
You now over subscribe based on the CIR. Doing some math we find that 1544/64 = 24.12 which means that with 24 customers you can guarantee that even if everyone runs their pipe at full bore they will all get their guaranteed 64kb. However in the real world BandWidth is expensive and and you never have all your customers using all their bandwidth at the same time so you can over subscribe by X times. If you wanted to be really nice to your customers you might start at 5 times OverSubscription. That means you can sell 120 accounts before your T1 is full.
From an ISP's point of view OverSubscription is the only way they make money and keep prices competitive. From a CommunityNetworking point of view OverSubscription is how you keep your members costs as low as possible.
The final question is how do you do this? Well the answer is you need a router (typically a Cisco but it doesn't have to be) which understands how to do QoS (Quality Of Service) definitions. If you want to bust out the elbow grease you can also do this with a Linux box, netfilter and the 2.4 kernels.
[Note: Bear in mind that with the charging of money for access you are really transitioning from a Community Network (where the idea is free and open access) to a Wireless Isp or CoOp. You might want to see ForFreeVsForProfit or WhatIsFreedom for more thoughts on the matter.] --AdamShand