The Hub is one of the six units of the M5GO, which can be purchased separately. This unit is used as a Grove door expander.
Attention: The Hub is just a physical expander, if you are looking for a software expander (standard protocol) like I2C, single bus, check PaHUB or PbHUB. If you connected two I2C slave devices to the HUB that share the same address, you will have problems getting I2C data from both.