Groupie

Assign users to specific user groups upon registration. Especially useful for front-end signup forms.

The plugin allows you to choose which user groups you want to allow members to register into, via front end registration forms. You can configure it to allow for registrations into multiple groups or restrict it to one group.

Groupie is quite easy to setup. Just configure the appropriate settings in the Craft CMS control panel.

In your front-end users/save-user form, just make sure one of your POST variables is groups. It can either be a string or an array.

A hidden field would work fine, or a select option or even checkboxes via name="groups[]".