Auto Actions Trigger List and Descriptions

Any use cases I can look at for changing profile field, or changing usergroup for a specific user via a URL link (with their username specified in the URL)?

You don't want to be doing that to be honest. That's dangerous unless you plan to secure the URL with some sort of special access token. The below would work and includes some very basic security.

Global
Triggers: None
Type: Usergroups
User: Automatic
Access: Everybody
Conditionals
1: [get_token] Equal To ACCESS_TOKEN_HERE
Action
Mode: Add Usergroups
Groups: SELECT_USERGROUPS_HERE

Next you'd use the URL provided on the Global tab to execute the action. However, you need to be able to specify who the target user is and include your security token. So the URL needs to have &users=USER_ID_HERE and &token=ACCESS_TOKEN_HERE and would look like the following for example.

index.php?option=com_comprofiler&view=pluginclass&plugin=cbautoactions&action=action&actions=ACTION_ID_HERE&users=USER_ID_HERE&token=ACCESS_TOKEN_HERE

The only problem is that I need to use username instead of userID. You said I can use substitutions inside the action? How do I do this, so that I pass the username instead of the userID?

You can't use username. It won't construct a user object off username. Why do you need to use username?

I need to pass the URL from an external CRM system, and I have a username convention that will allow me to pass that data from that external CRM. The Joomla userID doesnt mean much to anyone other than being a Joomla user ID. I need to identify the user based on either the username or another field on the profile (which I have a few of).

Is there no other way to id the user in the string?

Is there no other way to id the user in the string?

Nope, not unless you used a Code action and set the Method to PHP then in your custom PHP code loaded a user object from the username supplied in the URL (substituted with [get_username] for example).