Hi @Chris Ong
Thank you for asking this question on the **Microsoft Q&A Platform. **
Definitely, the way to allow third parties is to use Azure AD B2C.
You cannot merge the users between Azure AD and Azure AD B2C.
I had a similar case, and what we did was adapt the application to have one login page for Azure AD user (my company users); and another login page for Azure AD B2C (for third parties)
Hope this helps,
Carlos Solís Salazar
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