I think there’s also an Automation script that allows you to automatically add people to groups depending on the User Custom Field they select at signup (or whenever depending on how you set it up). For example:
Create the groups:
Create the User Custom Field:
Create the Automation:
Sign-Up:
See them added to the group: 
Fun quirk…
If you’re using the ‘Recurring’ trigger rather than ‘User first logged on’ you’ll need to use the user_field id rather than the user field name, but you can grab that from the relevant json if you need it /admin/customize/user_fields.json (in this case, mine’s 4)
NB: (note the format of the user_field_4)