If you want people to assist you on your event bosses, you must first fight them then your boss will appear on your "friends"' radar if you don't defeat them; if you want to assist your "friends" then the same has to happen for them first and then their bosses would appear on your radar.

For example if I were to fight a boss but lose the battle, then my boss would appear on your radar with whatever health it left after that particular battle.