Are you actually running five computers wirelessly at the same time?
An 802.11g wireless router has a nominal bandwidth of 54Mbps, which in practice will be closer to 30Mbps, and that is *shared* by the computers connecting wirelessly to it. You could just be overloading the wireless connection.
In general, there shouldn't be a limit to the number of computers connected. As far as Blueyonder are concerned, they have allocated a single IP address which goes to your wireless router.
If you connect one of the computers by ethernet cable instead of wirelessly, do you still have the problem?