There used to be a list of you and your allies on your game's main page with a total power ranking but it got cut for reasons unknown.
Until it comes back you can only calculate manually.
Making the squad banner in your signature is also simple manual work.
Use any screen capture tool, cut it to the area you want captured, save it as a file, upload it to a network image share and paste the link in your user options on this forum.
Windows has at least two default tools you can use for that - either the Snipping Tool or using the PrtScn keyboard button to capture your screen and pasting it to Paint, then cropping it.
Send the image to any image sharing website, for example
Imgur. After the upload you'll get a share link that is surrounded by [img] and [/img] tags.
On top of the forum's page you'll see the Settings button. Go there, then to Edit Signature, paste the link you've got from the share and you're done.