Make sure to download and install the latest firmware on the Sena, and get the app for your smart phone, if you have one. The Android app is nice to have as it includes the manuals and such.
I have the 2 unit set and they link to each other easily, and to my Samsung G6. Also links to my GPS (cheap, Chinavasion "Rage") but I don't usually connect the phone and GPS at the same time.
When riding in a group, I have not had good success connecting more than 4 units, but they were a mixture of makes and models, so I was not surprised.
Connectivity is not perfect, but it is improving...