Hi there firstly I would look at your group, make it stand out from all the others eg Group Description, Group logos ( group cover picture ) create interesting posts and Group content to engage your new members 
, make sure your group is visible and you have the group topic tags done, invite your Facebook friends to Join your group. Join promotional Facebook groups for Promoting your Group in. Create a Facebook page for your group and link it to your group and use that to also promote your group 
It’s an interesting question
I think the problem is even harder in that you don’t just want numbers you want people who are interested in participating in the group
you invite all your friends and colleagues who are interested in the topic to join and to invite their contacts who are interested in the the topic
maybe you let people who are in related groups that you follow that you have a new group
maybe you make some memes that will enthral and attract people and include your group address
maybe you pay FB to sell you access to a relevant demographic
maybe you make sure you have a catchy name and just be patient, I found this group the other day where the admin had abandoned it but it had a catchy name she had 35 thousand members who had searched and joined even thought there was nothing there