Existing users can be assigned to a client in the Admin Panel under User -> Options -> Edit:
The first setting specifies which client the user is permanently assigned to.
The second setting specifies which client should be displayed to the user at the next login.
If you have activated SAML as authentication provider, you can define in the edit mask of the client (Client - > Options - > Edit) with which IdP this client should be connected. Thus, users who log in via this IdP always automatically end up in the corresponding client.