Grouping speakers are connected directly to each other, not via the network to ensure audio sync. Each speaker has to be close enough to each other to connect. If different speakers are connected to different access points within the same network, there is a likely chance that they won't connect to each other although you can see both devices on the WifiStream app.
If you are planning to group more than 2 speakers together to play music sync, please choose the "most physically centralized" speaker as the main speaker of the synced group.