Guest posting won’t hurt your website in any way. In fact, it is one of the more reliable organic ways of getting traffic to flow your way. You should put in extra effort to make said articles as great as possible. For many people, the articles will be their first impression of you. Make them count, and your audience will gradually expand.
Of course, there is also the potential issue of the quality of the site you’ll be posting on. You generally won’t want backlinks from shady sites, but given that they are building a library of organic guest posts, I’d say that you’re in the clear.
However, a post a day does seem excessive. How long are you supposed to keep it up? You should make sure that it doesn’t negatively impact other areas of your work. You can only spare so much energy, after all.