I was in multiplayer with my nephew and he was creating a fleet and adding members too it. He was in a ship and had a ship parked in front of him 30m away and couldn't add it to his fleet. He had to ram into the ship to add it to his fleet, he then had other ships he was not able to add. I was not able to help him. Later in the day I was playing with fleets in single player and ran into the same problem with a ship parked about 50m in front of me. After about 15 minutes it dawned on me that there was a sector border between me and the ship. It is confusing when ships are listed in your navigation menu, and are right in front of you, but not listed in your add to fleet menu.
This is probably not technically a bug, because I believe it's working as intended... but I'm reporting it as a bug because it had me and my nephew stumped. And will probably stump and confuse other people too.