Hi Writer Girl,
Yes I don't think there is any doubt that you are the copyright owner in this case. That applies under both US and UK law. Be careful that anything you do sign does not include an assignment of copyright to the founder. I'm not clear why a non-disclosure agreement might be necessary in these circumstances but it sounds as if the founder is trying to bind you into a contract which may not be in your interests. Obviously you should be seeking a written agreement that recognises the verbal agreement over your future stake in the enterprise.
When you say that the online publication is based in the US I assume that you mean this is where the founder is based, as well as where the servers are based. In that case, it would seem likely that US copyright law could well apply and so it is worth clarifying the circumstances when the US doctrine of work-for-hire applies:
Source: 17 U.S.C. § 101
(1) a work prepared by an employee within the scope of his or her employment; or
(2) a work specially ordered or commissioned for use as a contribution to a collective work, as a part of a motion picture or other audiovisual work, as a translation, as a supplementary work, as a compilation, as an instructional text, as a test, as answer material for a test, or as an atlas, if the parties expressly agree in a written instrument signed by them that the work shall be considered a work made for hire. For the purpose of the foregoing sentence, a “supplementary work” is a work prepared for publication as a secondary adjunct to a work by another author for the purpose of introducing, concluding, illustrating, explaining, revising, commenting upon, or assisting in the use of the other work, such as forewords, afterwords, pictorial illustrations, maps, charts, tables, editorial notes, musical arrangements, answer material for tests, bibliographies, appendixes, and indexes, and an “instructional text” is a literary, pictorial, or graphic work prepared for publication and with the purpose of use in systematic instructional activities.
Clearly the only part which comes close to your situation is the 'work specially commissioned for use in a collective work'. But since you have not signed a written instrument to this effect you are certainly not bound by any work-for-hire agreement. Furthermore, the courts in the US apply a further test in order to determine the employer/employee relationship. The closer the relationship, the more likely it is that a work-for-hire situation might apply. The factors they look at include:
Source: Community for Creative Non-Violence v. Reid,490 U.S. 730 (1989)
the hiring party's right to control the manner and means by which the product is accomplished. Among the other factors relevant to this inquiry are the skill required; the source of the instrumentalities and tools; the location of the work; the duration of the relationship between the parties; whether the hiring party has the right to assign additional projects to the hired party; the extent of the hired party's discretion over when and how long to work; the method of payment; the hired party's role in hiring and paying assistants; whether the hiring party is in business; the provision of employee benefits; and the tax treatment of the hired party.
Once again this test clearly fails to show that something approaching an employer/employee relationship might exist. You can find more information on work-for-hire in this US Copyright Office circular
So having established that I think you unequivocally own the copyright in your work, how does this effect the relationship with the online magazine? Clearly you have licensed the magazine to use your work, and the implication of that licence is that they may use and retain your work for the purposes of maintaining the integrity of the site (just as would be the case if your words were published in a paper-based magazine). But they do not have exclusive rights to your work, unless this was specified in a license (preferably written, but a clear verbal agreement would suffice), so you can re-publish your writings elsewhere, but unless the founder consents, you can't insist he removes your work which is already in the online publication.