Alright ready:
1. Go to Admin Control Panel
2. Select tab "Users & Groups"
3. On the left, under "Groups", select "Manage Groups"
4. Once there, click the small people icon next to whichever group you wish to add people to
5. Once here, scroll down until you find the white box you can type in, type the person's username you wish to give this position, and click submit.
6. Once they appear in the group, there is a tab box that should say "select option" at the bottom right of the list of users. Click this, check off a user in the little checkbox by their name, and select "make group default for member".
This should put them in that group.