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).