GlassWire breaks down network usage in detail by traffic, app, host, so yes you’d need to install it on the PCs where you would want these details.
If you just want to know what devices are online you can just use GlassWire’s “Things” tab, and you don’t have to install anything on other PCs.
GlassWire lists devices with static IP addresses. Is it possible those devices are on a guest network, or you are on a guest network that’s segmented from them? GlassWire can’t see devices on segmented networks that GlassWire itself cannot access due to segmentation. For example a “guest” network.