With SharePoint modern sites, the default granularity options for accessing SharePoint content are as follows: 

• Full Control, which is granted to Owners
• Edit, which is granted to Members
• Read, which is granted to Visitors (guests)

It is recommended that you don't delete the default SharePoint security groups, though you can change their permissions and add new permissions levels if necessary. To customize the permissions for a site, follow these steps:

1. As a site owner or administrator, navigate to the site where you want to customize permissions.
2. Click the Settings (gear) icon on the menu bar and select Site permissions.
3. Click the Advanced permissions settings link. This will open the classic SharePoint permissions dialog box, as shown in the following screenshot:

4. Click Create Group to create a new SharePoint permissions group.

5. From this page, you can edit the group name, owner, and leave/join requests:

6. You also have the ability to choose a custom permission level:

7. After you have selected your options, click Create.

After the new permissions group has been created, you can begin adding members. While you can use a mail-enabled security group that's been created in Exchange or the Azure AD portal for many SharePoint tasks, you cannot use a SharePoint group in other parts of the service.