You can add Google Drive documents by sharing the link from Google Drive. However, you'll need to also make sure to go into Google Drive to make sure the file you choose is accessible to anyone with the link.
Step 1, in Google Drive:
If you aren't sure that the permissions of the Google Drive document you attached will allow people with the link to View or Edit, then go to drive.google.com, and find the file you added
Select the file within Google Drive
Click the "three dots" menu at the top right, then click "Share"
Then, click "get shareable link" at the top
It will default to "anyone with the link can view"
Set it so that either "Anyone with the link can view" or "Anyone with the link can edit" at your preference
Click Done
Next, copy this shareable link.
Step 2, in Hylo:
Create a new post in Hylo
Paste the link to the Google Drive file from Step 1.
Complete your Post in Hylo, and click the "Post" button.
If you don't do both steps, fellow Members may be able to see you attached a Google Drive file via the link, but they may not have permission to view it.
You can choose different or more restrictive approaches in Google drive for sharing the file and it will carry through to Hylo.