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Hi @Samyrah Gonsalves , welcome to Q&A forum!
Per my understanding, the current situation may be:
1. Your company already has a site, now you want to create another site as an extranet in the same tenant:
In this case, you only need to create a site according to the normal process, most of which are the same users in the tenant, and you can directly access the site as long as you directly grant permissions.
Reference:
Create a team site in SharePoint
2.Your company already has a tenant, and now you want to use a new tenant as an extranet:
If this is the case, it is a little more complicated and requires External sharing.
Please refer to these related articles for more information:
External sharing overview
Manage sharing settings
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