"We will start a group."
Translation:Vi ska starta en grupp.
skal is totally wrong since it means shell.
skall is an older and more formal version which is accepted in many places, including here, but most people use ska even in formal writing today so in real life I'd recommend never to use skall at all.
For de, the most normal way is to write de but pronounce it dom. The spelling dom should be accepted everywhere (if it isn't, just report it – except if it's a listen-and-type exercise, because there's a problem with getting the machine to accept spelling variation in those). But most people actually write de, not dom, so that's what we teach in the course.