We have a missionary in our ward who has been here for 9 months now. He's really gotten to know the area and the people in our ward. What he's been doing is helping us as members feel comfortable inviting our friends to church and church activities.
October, November and December are big holiday months with lots of church activities going on. Perhaps your missionaries can help make those activities one that the members can be proud to invite their friends too. Have them get the word our that members are supposed to invite friends. Get members excited and committed to the activities, to making them a success and something they would be proud to invite friends too. You've seen that missionaries on their own aren't doing much, but they can help support the already active members be more successful missionaries.