The AGPL does not define the term “user”, but context makes it clear (to me) that this means persons, in particular natural persons that are end users, or other third parties.
As Brandin points out in the comments, the precise definition of users is not that important. The AGPL's requirement to offer the source of a modified version effectively triggers as soon as a user could interact with the software over a network: you have to proactively and prominently offer the source code, not just respond to requests. Even if your AGPL-covered software is intended to be used by other programs, this doesn't prevent end users from interacting with these APIs as well.
What constitutes interacting is left open in the AGPL, because the FSF typically wants to use the strongest interpretation that is available under local law. I think it is safe to assume that interacting with the AGPL-covered software through another program (like a client software or a proxy) is still interaction. The offer of the source code would not have to be made to these client programs, but to the actual user.
On the other hand, the AGPL only covers interaction, not other use. For example, a user who interacts with a web app which in turn uses an AGPL-covered modification of the MongoDB database: does the user now interact with that database, thus triggering source disclosure requirements? No: in the general case, the web app's provider will be the user of the AGPL database. The end user does not meaningfully interact with the database at all. But of course that would depend on the functionality of this web app. For example, if the web app is a database management interface, that would probably not prevent any interaction.
So whether a software between the user and the AGPL-covered program prevents interaction in the sense of the AGPL depends on context. Unless it is plainly obvious that no meaningful interaction takes place, it is of course safer to assume that such interaction is in fact happening, and that you have to offer the source code.
Please also note that the AGPL source offer requirements do not trigger for most uses of AGPL-covered software, because they only trigger if you make a modified version of the software available to third parties. Running an AGPL-covered software by itself triggers no requirements, and neither does purely internal use.
