Ability to Self-Select multiple user groups when user is creating account

Currently, you can add a drop down list for users to self-select their own user group when creating their account. However, they can only select one group. Most of our users require at least two groups (The Everyone group and their team-specific group). Checkboxes with all available user groups would be preferred over a single-selection drop-down.

With this in mind, the ability to automatically add all users to a group (such as the Everyone group) in addition to their selection would be ideal.