I add a new student join. Any way that student can start at the same skill lesson as the others?
There's no way for you to let him skip parts of the course, no. Every Duolingo user has to progress through the course in order. If your student already knows some of the stuff, though, there are three ways of skipping some of the course:
Take the placement test when first using the course. Depending on how the student does, it may let them skip a few skills.
There are a few yellow checkpoints in the course. Each checkpoint has a test, and if you pass, it lets you skip the skills up to that point.
Each skill has a test out option. Again, there's a test, and if you pass it marks that skill as complete.