There are 2 options:
Send a "fill a form" request through a portal group
In this case all portal users who are members of the portal group will receive the request.
Prerequisites: you must define the form in "Form templates" section first
Broadcast a document for signing
Create a signing template (1 signer allowed only)
Send the signing template for signing.
Add signees in bulk using a group name (you need to define the group first in Company Users section)
Make sure you tick "Each signee signs a separate copy of the document" option in Advanced Settings
If you would like to have signed copies appear in client portals, make sure you assign the portal to each contact in Company Users section