The issue that's raised is that, generally, when friends play together, they instinctively ally with one another giving them an unfair advantage, or ca communicate in gunboat games, etc. if the whole map knows that you know each other and they choose to join your game, then it's not an issue.
I've played with plenty of friends, relatives, colleagues, etc before, however being MY friend in this game just means you'll get stabbed. So, just understand the nature that comes with having friends here. We do welcome invitees.