For an organisation with eight people, you have probably all the network you need right now, with the one switch. If you want to separate the traffic, so some people do not have any access to some resources, then configure vlans, and upgrade the switch to a Layer 3 switch. This will allow you to route between the VLANs (as each one needs to be a separate IP subnet), and also to apply some access control to restrict what networks can talk to each other. The modem can stay almost as it is already, but needs to allow all the sub-networks (or just the ones you want) to have access to the Internet.
I suspect you will also want to add a server for e-mail and file sharing, and probably a printer or two, so when looking at a suitable switch, go for at least 24 ports, which will give you some reasonalbe expansion capability. If it has a couple of Gigabit ports as well, then use at least one for the main server. The other can be used to link to another switch in the future, if the needs expand.
It doesn’t have to be complicated, in fact the simpler it is the more likely it is to work without problems.
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