I see, in this case there are several ways of saying depending on the person.
(私は)5分で始(はじ)めます。I'll start in 5 minutes.
(私は)5分で行きます。I'll go in 5 minutes.
(私は)5分で合流(ごうりゅう)します。I'll join in 5 minutes.
However, I think you can use the casual form with your friends, so I would say:
5分後に始めるよ / 行くよ！
After some conversations, like "see you in 5 minutes!" (じゃあ、)5分後に！
I hope this was helpful.