Here is a step-by-step guide to adding another signature to your collaborators' Gmail.
We will try to address the following use case in this guide:
My agency director thomas.dupond@agencea.com wants to have a signature for Agency A and Agency B on his Gmail.
Create a Google Workspace alias (Google Workspace Admin)
To be able to update the correct signature on Gmail, another user must be created on Signitic and therefore on Google Workspace. To create an alias, you will need to ask the person in charge of Google Workspace to handle it.
For our example, I will therefore ask the IT department to create an alias thomas.dupond-b@agencea.com for the user thomas.dupond@agencea.com.
Bring up the alias on Signitic (Signitic Admin)
Once the alias has been created in Google Workspace, we will check that the user is properly reflected on our Signitic account:
Check that alias retrieval is enabled on my connector
Run a user update in the emergency console.

If everything goes well, you should have retrieved your user on your Signitic account.
For our example, I am retrieving the user thomas.dupond-b@agencea.com.
Edit the alias (Signitic Admin)
Once added, you need to:
Verify and complete the information for the added alias in its user profile.
Assign the correct template to the alias group.
Add the alias to the user's Gmail (End User)
The final step takes place in your colleague's Gmail account, by following this guide, they should be able to add an alias in their Gmail, and their signatures will be updated by the next day.