Hi,
selectize does not support form-group bootstrap class. It breaks css formatting.
Please see this example
http://jsfiddle.net/5ewytn3n/
Use the inputClass option to add "form-control" to the class: http://jsfiddle.net/5ewytn3n/1/
I still have a problem with that

The above fix does make it line up for me, but only until you open the select. Then this:

@MarshallHouse This was driving me crazy, too, especially when I pasted in @flamber's markup and it worked. I went back and forth trying to spot the difference and it came down to not having the form-control class initially on the selectized element.
Here is a fiddle demonstrating the broken behavior, if you remove the form-control class, it works.
I hope this helps you.
@MarshallHouse @johnrork I believe the correct way to solve your particular behavior is to add dropdownParent: "body" to the options passed to selectize:
http://jsfiddle.net/5ewytn3n/13/
Thanks @rmm5t for saving me time & headache! Your solution works great with BS3 and bootstrap's input-groups form fields.
@rmm5t That works if you have a single select, but what if you have a multiple select? The input area doesn't expand. Any ideas for a workaround for that?
@ghiotion Not sure. Might be something else, because I have it working in a page with multiple selects.
both inputClass: 'form-control selectize-input', dropdownParent: "body" are necesary for obtain the correct visual result.
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Most helpful comment
@MarshallHouse @johnrork I believe the correct way to solve your particular behavior is to add
dropdownParent: "body"to the options passed toselectize:http://jsfiddle.net/5ewytn3n/13/