There are two possible ways to register multiple authorship of a work:

If the authors have a Safe Creative user, the user who has made the registration sends invitations to the other authors to register their co-authorship.

1.- In the work's file, select "Authorship and rights".
2.- Click on "Invite to register rights".
3.- Indicate the email addresses of the co-authors (they must have a Safe Creative user but a professional or business subscription is not mandatory). 
4.- Indicate if the co-authors will have access to the registration editing functions.
5.- Click on "Request registration".

Safe Creative will send an email to each of the co-authors with a link to confirm their authorship, which will appear -once confirmed- in the registration information of the work.

To send co-authorship invitations for several works at the same time, you can select all the works in the registration page and check "Manage selected". The operation is the same, from the "Invite to register rights" option.

If the registration is made as an authorized manager, on behalf of the co-authors (who may not have a Safe Creative account).

Adding the corresponding authorship registrations, either when registering the work or once it has been registered.