As we no longer have a user group in the session, the user will have to be presented with a user group selector when uploading a file.

Probably the choices of user groups in that selector should be limited:
1) To the user groups assigned to the user.
2) To the user groups that make sense in the context of the upload (e.g. reviewers for reviewer file upload, authors/press managers/series editors for submission file upload in the submission process, etc.)

When there is only one user-group that fullfills these filter criteria then we don't need a selector but can set the user group by default.

When there are several user groups available then we could select a gould default like this:
1) If we are in a workflow page context we could set the (first) user group assigned to the user for the current workflow step as a default.
2) Otherwise we could set the user group with the lowest row id as a default (e.g. press manager role overrides author role, etc.)
3) Only if these heuristics do not provide a good default could we choose the first user group in the list.

I propose that we order user groups alphabetically in the drop down.

A similar logic will have to be implemented for event logging, see