Hello Gabriel

This already exist, you can assign the users to the group:  Sales / User: Own Documents Only
In this group they will only be able to see the records related to them

However if you want to implement this in a new group
you can create the group then create a record rule like this:

Select all checkboxes

Name:  "Leads: Own Records or whatever suits you"

Model: ​"Lead/Opportunity"

Rule Definition (Domain Filter): ['|',('user_id','=',user.id),('user_id','=',False)]

Groups (no group = global): "The Group You created"

also add the record rule: "CRM Lead Multi-Company" to the group you created to make sure they only see the leads related to their company

then add users to this group and make sure they are not in another group which have more privileges than this one