Website management for group seminar
A site hosted by your university, on your university website, will probably be a better choice than an externally hosted website. This is because you'll be able to immediately identify the seminar series with your university, and that will help to improve its branding. (It also looks a lot more professional!)

As for software, there are a lot of different options. I can't really offer a lot of guidance on this, as we have staff whose job it is to maintain our websites. Which one you pick will depend a lot upon the kinds of features you want, and how steep a learning curve you're willing to negotiate.