Open any content in a Bootstrap pop-up like the one we use on this site. Go ahead, check it out. Click the login link at the top of the page.
Inserting the link to open the modal is a simple as inserting a shortcode on your page. Insert [frmmodal-content label="Click me" class="extra classes here"][formidable id=25][/frmmodal-content] for a form or [frmmodal-content label="Click me" class="extra classes here"][display-frm-data id=x filter=1][/frmmodal-content] for a view.
Insert the login form shortcode and any other shortcodes with [frmmodal-content label="Click me" class="extra classes here"]Put shortcode here[/frmmodal-content]. Replace "Put shortcode here" with any shortcode you would like to insert.
Open any other content, including shortcodes from other plugins, with [frmmodal-content label="Click here"]Add all the content you want including shortcodes from any plugin.[/frmmodal-content]
Note: If you are planning on using the Bootstrap Modal add-on with the Lite version of Formidable, you can include forms in modals, but the modal will immediately close upon submission, even if there are errors. It works best to use the Pro version of Formidable so the "Submit with AJAX" option can be enabled in the form's Settings.