How can EB-5 dependents attend a visa interview if the principal applicant already gets a green card?

She can file an application to join since you are in the U.S. and have obtained your permanent residency. She must submit documents showing the relationship between both of you, particularly the marital relationship and all associated information included in the underlying I-526 approved petition.

Yes, she must schedule an appointment and file the DS-260 application.

You should have included an I-824 with the adjustment filing, although there may be other options to process a derivative following to a joint application. If you have not filed, file it now.

If you applied in the U.S., she will need an approved I-824 first for the consulate process.

Yes, she can. She will need to provide copies of your approval notice for I-526, your I-485 approval and a marriage certificate, along with her supporting documents.

That is the classic "follow to join" process. An immigration attorney can definitely help with that.

Your wife needs to complete the "follow to join" process. Your attorney can initiate it with USCIS.

Yes. This is called a follow-to-join. She needs to provide all of her civil documents, including birth certificate, marriage certificate and police clearance certificate, and show a copy of your green card. You may need to file a Form I-824 to request USCIS to notify the consulate in your home country of your I-526 approval and adjustment of status. That will start the immigrant visa processing of your wife.

It seems to me that you need to submit a form I-824 notification.