This article is about a concept and an object in SAP EWM called the availability group. To be honest, I would have put the availability group in the organizational structure section, but SAP itself assigned it to the goods receipt processes. To understand what an availability group in EWM is, you first need to understand the organizational structure of SAP ERP.

In SAP ERP there is a plant, and storage locations are assigned to it. According to best practice, SAP ERP has at least two storage locations – ROD (Receive on Dock) and AFS (Available for Sale). The first one is used to record goods that have been received into the goods receipt area of the warehouse but have not yet been put away into storage bins. The second one records goods that have already been put away into storage bins and are therefore available for sale.
So, the availability group in SAP EWM is a storage location in SAP ERP. Hence the unusual name – «availability group» – because in essence it indicates whether the goods are available for shipping or have only just been unloaded at the warehouse door. In practice far more storage locations are used than the two given here as an example.
The availability group is defined in SPRO:
Extended Warehouse Management - Goods Receipt Process - Configure Availability Group for Putaway - Define Availability Group:

You specify the warehouse number, the name of the availability group and a description.
The mapping between the ERP storage location and the EWM availability group is maintained under:
Extended Warehouse Management – Interfaces - ERP Integration - Goods Movement - Map Storage Locations from ERP System to EWM:

For every plant / storage location combination of a particular ERP system (identified by the logical system), you specify the warehouse number and the availability group in EWM. The party entitled to dispose is determined automatically on the basis of the plant. That is essentially all there is to the availability group settings.
So where is the availability group used? Mainly to determine the stock type of the goods. What a stock type is and how it is determined will be covered in the next article.