Don’t bribe people to join your community. No freebies for registering. Instead decide what the problem is and change your approach.
Your audience isn't joining your community because either:
a) Your audience doesn’t know your community exists.
b) They know your community exists, but they don't feel like joining.
If the answer is a), you can begin building relationships with people who can promote your community in the future. This takes time, so spend time doing it. Leave two comments on his forum today, offer help to a top blogger. Better, run events designed to attract attention. You can offer rewards for existing members who invite friends.
If the answer is b) then you need to make the community more relevant to your audience. This means bringing your community, or creating elements of your community, which are more closerly aligned with your audiences' needs, goals and daily life.
You might create a group for his/her local area or industry. You might interview people your audience admires. You might see what they’re discussing elsewhere and bring those conversations into your community.
If you're still determined to bribe, don't bribe members to join. Bribe for participation.