you go to I/O -> Group -> F2 (CONFIG) and then (if i recall correctly) you have to assign RACK, SLOT, START PT and NUM PTS parameters for each Group Signal.
RACK, SLOT and START PT have exactly the same meaning as for DI/DO configuration. NUM PTS is just a number of bits - it tells the robot how many bits (starting from the first, pointed by the Rack, Slot and Startpoint) are used to code this particular Group Signal.
So, for example, if you configure the GI[1] as RACK 89, SLOT 1, START PT 1, NUM PTS 8, you will get the first 8 digital inputs of the EthernetIP device (configured in EIP connection 1) to work as an 8-bit integer, available to you under GI[1].
If these DIs will then be set to:
# 1 0 0 0 0 0 0 0, the GI[1] will be set to 1.
# 0 1 0 0 0 0 0 0, the GI[1] will be set to 2.
# 1 1 0 0 0 0 0 0, the GI[1] will be set to 3.
etc...