Rather than using a list of email addresses, it would probably make more sense to use the API for this. So, when the user signs up, you should automatically enroll them in the group using this API: Discourse API Docs
That requires that you also have the user already created in Discourse at the time you use the API. This might require a bit more setup, but it could completely automate the flow for you so you aren’t doing manual uploads of user email addresses.