How to launch Trello on Login as menubar item?

Yes you can. Go to the Trello App and click on Preferences. Clicking preferences brings up a pane as shown below.

Under the phrase Window Style click on the pull-down menu arrow showing the pane below.

Click o menu only to have Trello show as a menu icon only.

Now you need to get it to start on login or restart. To do this go to system preferences under the Apple menu item. Click on the "User & Groups" preference pane. When that panel appears, be sure to click on the tab "Login Items". This displays a listing of items started on Login. Be sure to unlock the panel and click on the + button at bottom of the list. This allows you to add an app to start at login. After clicking the + button locate Trello and select it. This tells Trello to start at login. See image below.