Abis Grooming is when you use one E1 to groom more than one site together, By meaning the 32 channels of 1 E1 are divided for more than one BTS.
For example if configuration of 2 sites is only 900 1+1 than it only requires 4 channels for TRX and 1/2 Channel for OMU signaling and 1 channel for TRX Signaling, assumin 32 bit for all signaling channels.
So we can use the remaining to groome (Add) another site to this E1.
This is not recommended unless you have no capacity on your PDH link.
Usually used for road sites.