You can add contacts on your account at explore.garmin.com and sync them to your device. You can also add contacts on the device, but those contacts are not synced with your online account and therefore not backed up.
This article explains how to add contacts to your inReach.
1. Select Contacts and press
2. Select New Contact and press
3. Select any of the contact fields and enter the details.
4. Select Done and press when finished.