Bring up the Sysadm client (what you call control panel), go to Users.
You should see 2 tabs, Users and Groups
Click on the Groups tab
Left hand side should have Standard and Advanced, right hand side should have Users with 2 columns, Members and Available.
On the left had side, click on Advanced, that will give you the whole list of groups. Scroll down look for webcamd, and select it.
Now look to the right hand side, Members is probably empty, look for your username in the Available list and select it.
On the right hand side there are 2 buttons, one under Users, pointing right (says remove if you hover your mouse over it), one under Available pointing left (says add if you hover over it) . Click the Add button and you should see your username appear in the Members list. Click the Save button and you should be all set.
@RodMyers command is almost correct. Since your user already exists it should be:
sudo pw usermod xxxxxx -G webcamd
the command he gave will try and add a new user, since you already exist it fails.
man pw will show the manpage (help) for the pw command he used.