Addressing a large audience of (close) friends: Casual or formal or both?
@philg - My experience and observations are as follows:
1) When addressing an audience from a stage, it is common to use 존댓말 regardless of whether they are older or younger.
2) When talking face to face with a group of friends, some of whom are older and some of whom are younger, it is common to switch back and forth between 반말 and 존댓말 when speaking, depending on who you are talking to or who you are addressing specifically. It is, for example, not uncommon to hear someone switch between different levels of politeness as they look around the room or around the group of people they are with.
3) When posting on social media platforms where multiple people will see it, it is common to follow the same protocol as with 1) and default to 존댓말.
As a non-Korean, friends will be more forgiving. In general I try to err on the polite side, as people who use 반말 too quickly can be viewed as lacking respect for others and for elders.
In the case of your specific example, you can increase the politeness by using "시":