Two things makes inviting new members much easier: 1) If they know you and 2) If they receive the invite from a friend.
If someone doesn’t know you, don’t invite them to join your community. The response will be poor, the members will be less motivated and most wont stick around for long. The less connected a newcomer is to your community’s existing members, the harder you have to work. Too many strangers and you’ll struggle.
You have two good options to invite people to join your community.
1) Build a relationship first. Don’t invite a stranger at the first message. Instead, explain that you have the time and resources to build a community for him and his friends, what does he think would be best? Work out the details. Let him take ownership, give him influence and you’ll find he invites his friends along too.
2) Find others to invite their friends. Invest the time on existing members. Give them ownership, influence and that feeling of pride. Then create a nudge to invite friends.
Stephen Covey would call this sharpening the saw. Do this in the beginning and the process is much better.