The meanings of 'join' and 'join in' are different.
'Join' is transitive, which means that this verb has to have an object. You can join a club, join a team, join a company, or in the case above simply 'join us'. We always need to know what or who you are joining.
'Join in' means to participate in an activity which is already going on. We use this when several people are already doing something (playing a game, having a party/a conversation/a fight etc) and other people start doing this thing with them. 'Join in' doesn't have to have an object - you can just say 'Do you mind if I join in?'