Here’s a quick walkthrough of how to set up an Automation using the Add user to group through custom field option 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):
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)
And that should be that. 
If you have any questions please ask them below or spin up a new support topic. 