How to add user group to external users

You can create a new user group for external users but there's no built-in way to automatically include external users to that group. So your choices are either to maintain the user group memberships manually (suitable for a small vault, where users do not change that often) or develop event handlers / VAF application to do this automatically: whenever a user is created or modified, check if they should be added to or removed from that user group.