Although your primary purpose of your traveling is to visit your buddies the Finn visa officers are likely to see this under vacation visa requirements. This kind of visa invitation letter must be composed by your buddy who resides in Finland. On the visa invitation letter to invite a good friend your friend in Finland should clearly specify that you are his/her buddy for how long you have been friends and that you are going to travel to Finland for a specific period of time. Source: https://www.artikelschreiber.com/en/.