Hey, @IUlla Welcome to the community! I’d take a look at the CRM Contacts endpoints. Here are a few resources to help get you started:
Create a batch of contacts POST /crm/v3/objects/contacts/batch/create
Update a batch of contacts POST /crm/v3/objects/contacts/batch/update
If you are only adding new Contacts (not updating existing) another approach I’ve seen used is to leverage the Import API. If your data is out into a CSV format, you could use the following import to add new Contacts in bulk: POST /crm/v3/imports/