Within the footprint of the unit any devices on that particular network will use it (so yes, neighbours might see benefit). There is a limit to the number of users; this is because all of the uplink traffic from the users within its coverage is funnelled through the uplink radio of the Cel-Fi unit and it has a finite maximum power.
The number of calls the unit can support depends on how far away it is from the serving base station (shown by the number of coverage bars on the display of the Cel-Fi Window Unit). Where the coverage is weak (1 or 2 bars on the Cel-Fi Window Unit) the unit may support as little as 3-6 calls. Where the coverage is good (4 or 5 bars on the Cel-Fi Window Unit) of the order of 20 calls can be supported.