The group discord is used for voice chat, general meme spam, and text chat even if you aren't friends with the other members.

About hidden/private profiles: Other group members will not be able to join your games from steam unless you directly invite them, which would then require them to be on your friends list. If you don't want your friends list to become clogged because of the group, I recommend setting your profile to public. Then, you can set your games as "Friends only*", and any player can join your non-private online games from your profile.

*Friends only in PD2 does not actually mean friends only. Anyone who can view your profile can join your game under this setting, but your game will not appear on Crime.net.