A group (or role) is a collection of users categorized by their job titles and job roles, not by the departments they belong to.
For example, you can create groups such as "Sales Managers" and "Recruitment Team A" based on their activities.
Services in Which Groups Are Available

Groups can be used when setting access permissions or configuring recipients of comments and notifications.

You can configure a membership type when creating a group. You cannot change the membership type afterward.

A static group is a group whose members you need to specify.
Changing the Members of a Static Group

A dynamic group is a group that you specify conditions, such as job titles or departments of the users.
When you set conditions, users who meet the conditions are treated as group members. For example, you can specify the conditions that a user who is a member of the department "Sales Department" and has the job title "Managers".
Specifying Conditions for Dynamic Groups

When the settings of the department to which the user belongs or their job title is changed, the members of the dynamic group are also changed accordingly.

The maximum number of group members is 10,000.

The maximum number of "Administrators" group members is 10,000.