To encourage your community members, you may want to consider these suggestions:
1. Create at least 10 bot users: It improve the sense of engagement inside your community. You are going to need it a lot once you have your very first members on board. Use these bot users to up-vote others' answers, and to post content on behalf of the bot users.
2. Use the Welcome Email feature: By enabling welcome email, an email will be automatically sent to your new community members. It improves the first impression and helps your community members to learn more about the culture/policies/... in your community. Please check out this link: Automated Welcome Email2.
3. Follow your new members: Your new members would like to be noticed by the community managers. In the early stages, we suggest you to follow all your new member. To find the newest members, go to Admin Panel> Detailed Reports> Users> Creation.
4. Request An Answer from your new members: Post interesting questions and request an answer from your new members, or the experts who are able to provide you with the best answers. This way your members get notified that you asked them specifically to contribute in providing an answer. Check out this link: What is "Request Answer" on Tribe and how does it work?2
5. Mention the new community members in your comments: That would be very impressive for the new members to know that you remember them and you mention them when you find something useful for them. You can mention someone using @ before his/her name or username.
6. Make a competition inside your community: Define rewards for a question and give prizes (it can be a discount for your products/services or a gift card or anything else) to the best answers. Check out the link below, explaining how to define rewards for a question.2
7. Give a verification sign or a badge to your special members: Special members can be experts, well-known people in an industry, or simply any member who is very active in the community. You can make an announcement on your community and mention that the most active and engaged members with more than a specific number of the points will be a verified user or a member with specific badge.
How to verify users on my community?2
How to give a badge to a user?2
These are just a few tips you can use to encourage your members to be more active inside your community. We will post more suggestions late on. Please do not forget to make a comment under this answer if you wan to add something.