ASx transfers are "1-sender, 1-recipient" affairs. Although support for multiple recipients is common in similar encryption schemes such as PGP and SMIME, ASx transfers do not support this concept.
Most ASx products define the two sides in a file exchanges as "my organization" (i.e., you) and "your partner" (i.e., anyone else). Both sides are responsible for coming up with at least two pieces of information: an SSL certificate and their trading partner name. (In practice, one side may provide all of this information.)
The two sides must exchange their information (minus the private keys on their own SSL certificates) and agree on which certificates and trading names before any ASx file transfers may take place.
Each AS1, AS2 or AS3 protocol will also require each side to agree on additional protocol-specific items such as which server to send files too, what credentials to use to authenticate to the server and what flavors of MDNs are supported. Additional information about these specifics is covered in each ASx protocol's discussion.