From the community homepage, click on the competition in which you'd like to add participants to.
Click Actions, and then Edit.
Select the appropriate division.
Under team/participant, there is a dropdown box for each team row, click into this and select the team/participant you wish to add. Repeat this until all teams/participants have been added. If there aren't enough participant spaces, simply alter this by increasing the number of participants at the top of the page.