"Clients"  section is for adding a client and organizing a group of your clients. 

In this example, we will add "Margaux Doe" to Doe Group.

1. Select "Clients", then type it either name or surname through the search bar.

2. Tick the name that you want to add in a "client group", note that you may select one or multiple names. Once ticked, press "Add to Group".

3. Search and select the group for your client

4. "Margaux Doe" is now added to "Doe Group"

Other way around, you may select "client group" via "Settings>General"

Related Article:

Client settings

Define Client Relationship

Add related entities / Adding Client