There are three ways to invite participants to take part in an online discussion. A. B. and C. are reviewed below.
A. You can first create a project list in your "Member Panel" Please review the video and section how to create a project list: https://support.groupquality.com/hc/en-us/articles/201171477-How-to-create-a-project-list
Once you have created a project list the list name will be available in step 1. when creating your discussion board. The section you are looking for is titled "Select project list". If you can not see your list here it means you may have uploaded participants to the member panel but you have not yet added them to a project list (please take a moment to review the tutorial).
More about the discussion board creation steps: https://support.groupquality.com/hc/en-us/articles/201171787-Quick-start-discussion-board-5-step-process
B. Another way of adding participants to your discussion is by selecting the "Intercept" option in Step 1. of the creation process. You can also use this in conjunction with the project list method. https://support.groupquality.com/hc/en-us/articles/203977896-Creating-discussion-board-intercept-registration-links
C. Once you have "Started" your discussion board it will sever the connection with the original project list, and to add additional participants you can use the "Add Participants" button in your boards "Participant" tab.
You can find out more and detailed instructions by reviewing: