A good way to build your list is to offer Forum Membership to you members.
You can manually give full access or selected category only.

To do this:

• In Manage Members Dashboard, to search for a member simply type the Name or Email Address and click on the SEARCH button.
• Once you filtered you desired member, on the right hand side, click on the Courses button.
•  Among the courses available, you can find Forum.
• A drop-down is available for the Forum where you can manage the access. You can choose between No Access, Full access or Partial access.
  • No access will completely restrict forum access.
  • Full access will give full access to all categories you set up on the forum
  • Partial access will let you select which categories you wish to give your selected members access to.
• In all cases, be it Full access or Partial access, you will need to select a Date by clicking on the empty box next to the access type and select a date that will mark the beginning of the access time.

Click on SAVE CHANGES and Forum access for the selected member will be updated.