Your members needs prompts to invite friends. But asking friends to invite their friends doesn’t always work. It’s a cliché and it looks selfish. There’s no real reason for them to do it.
So create a reason.
Launch a specific project their friends can help with. Or begin debates you need expert opinions on. Keep it relevant to your community. Try a sub-niche.
Now ask your members to invite specific friends who can contribute. Perhaps a friend who recently moved house? Or a friend who’s just gone through a divorce? Maybe someone that works as a teacher.
You will hopefully get motivated members with something to offer your community.