Cant add {JFBCLogin} in the "Join Group" pop-up window

That popup is loaded specially by JomSocial and doesn't call the system plugins, which are used to parse/replace the {JFBCLogin} tag. To add the button there, you'll need to use the alternate method of adding a Login With Facebook link, where you can use custom text or a custom image. You can see how to do it in question #2 on the page below:
www.sourcecoast.com/jfbconnect/docs/common-support-questions

Hope that helps, but if you need more assistance, just let us know!
Alex