How can a poster of a comment notify selected users about a new comment?
When I write a comment on a node, I want to be able to select the users that will get a notification about that new comment.

1) Download Rules and Entity Reference Modules.
2) Enable Rules, Rules UI (User Interface, we need this), and Entity Reference
3) In your comment fields

Add a field with Entity Reference and a widget of Auto complete (tag style)

For example: I named mine lalala (click picture below to enlarge)

4) In your field settings:

Select User as your target type


After you click save field settings you are taken to another screen, scroll down and find Number of values and change it to Unlimited and Save Changes.

5) Add a new rule by going to /admin/config/workflow/rules/reaction/add

React on Event: After Saving New Comment
Restrict by Type: Select your content type (In this example it's Article Comments, since in Articles Comments is where I put my lalala field.)


6) Click on Add a Loop

Data selector: comment:field-lalala (remember I named my entity reference field lalala)


7) Click Add Action, but the one from the loop!

8) Select Fetch Entity by Property

Entity Type: User
Property: Email
Data Selector: list-item:mail
Limit Result Count: put a large number here in case it has an effect. (since we are looping, I don't believe so, but I am not 100% sure)


9) Add another action to the loop, this time add Send Mail
Click on switch to data selection button.

To: list-item:mail

The end result should look like this

Now add a new comment.
On your entity reference field since we are using Autocomplete (tag style) you start to type a user name and select the user. Then add a comma , to add more users
Ex: 
After posting the comment, all referenced users should receive an e-mail.