Taking Objectives Mid-Late Game

I'm having some trouble convincing my team to group up around objectives. There have been a number of games where my team will keep farming their lanes/sit in the jungle when we have an advantage we can use to take a dragon/tower etc.

I've tried asking thing like: "Let's group mid to get this tower", but it's usually ignored and results in the enemy being able to group and take an objective, despite having a disadvantage.

Tl;dr: How do I get my team to group up and take objectives?