Login to the dashboard, click "users and groups", search for the user in the list, and click on their nickname. Then click "Edit User". The page will reload and you should see "groups" with a blue "add group" link on the righthand side. Click that and an overlay should open up that lets you select a group. Select the groups you want, then save the user.

Andrew, bit of a hijack, but i'm running 5.6.0.2 and get the following error when trying to add a user to a group, can you help?

Warning: Cannot modify header information - headers already sent by (output started at /home/hawkwell/public_html/football/concrete/core/controllers/single_pages/dashboard/users/search.php:171) in /home/hawkwell/public_html/football/concrete/core/libraries/view.php on line 901

It says the user updated successfully, but it has not.

Thanks