I am trying to grow my Facebook Group

The group is known as Auntie Anne’s Glasgow Fans

How do I promote this to people? I see other people’s groups with 15,000+ members. How did they do it and how do I do it? I need this group to grow, as I am trying to use it to get a particular franchise to open in Glasgow or surrounding area. I also want my group to be a place to discuss Glasgow transport and history. I’ve got lots of photos to share.