Resource Groups help to organize resources, e.g. for Reminders, Daily Time, etc..
A resource can only be assigned to a single resource group.
The resource group is a Picklist field defined on the Resource object. To add or manage Resource Groups:
Classic:
> Setup > Create > Objects -- Click on Resource
- in the Custom Fields & Relationships section -- Click on Resource Group
-- in the Values section, edit existing groups, add new or delete/merge an unused group
Lightning:
Click on the Gear Icon in the top right corner and Click on Setup
- click in the Object Manager and search/click on Resource
- click on Fields & Relationships
- click on the Resource Group field
-- in the Values section, edit existing groups, add new or delete/merge an unused group
Note that in some areas , the Skill Set can be used for resource selection too. The Resource Skill is a multi-select Picklist (i.e. a resource can be assigned to several selections).