When an alliance is set to “Invitation only”, anyone who meets the critieria to join may request to join and at that point they may be invited in to discuss the details.
Yes, you can boot someone from your alliance at any time and eventually they will be deleted from the field of battle and the roles. I am pretty certain the points already awarded for defeating the person will stay in the final results, but I do not think they can be attacked after they have been booted from the Alliance.
You can’t delete the from the battlefield while staying in the alliance. .in fact if you could, it wpuld lower your chances of victory as the rest of your teams would be worth more points and they would need less flags to capturen the points