When you already have existing School/Groups and you want to add members to them, you will need to mention appropriate School and Group Ids.

To download them. 

1. Go to District

2. Click on District Settings

3. Click on Edit Settings

4. Click on BulkImport tab, and download School and Groups list

Sample files

Group File  (The ids mentioned are are for reference only, your groups will have diff ids)

• Use group_sis_id field value if you opted to go with sis_id in Import
• Use group_id field value if you opted to go with groupid in Import

School File (The ids mentioned are are for reference only, your schools will have diff ids)

• Use school_sis_id field value if you opted to go with sis_id in Import
• Use school_id field value if you opted to go with shoolid in Import