Yes, you can throw a weapon and teleport it back to yourself during an attack using Weapon Bond.
The rules for bonus actions state:
You choose when to take a bonus action during your turn, unless the bonus action’s timing is specified, and anything that deprives you of your ability to take actions also prevents you from taking a bonus action.
Therefore, nothing prevents a character from using their bonus actions during another action, such as an attack action.
The Eldritch Knight's weapon bond feature states:
Once you have bonded a weapon to yourself, you can’t be disarmed of that weapon unless you are incapacitated. If it is on the same plane of existence, you can summon that weapon as a bonus action on your turn, causing it to teleport instantly to your hand.
Therefore, since the Eldritch Knight's weapon bond doesn't require any special timing or conditions, other than the weapon remaining on the same plane, and the teleportation effect is instantaneous, they can use their bonus action during their attack action to summon their weapon back to them.
Yes, there's nothing (in the rules, anyway) preventing multiple Eldritch Knights from bonding with a single weapon.
Once you have bonded a weapon to yourself, you can’t be disarmed of that weapon unless you are incapacitated. If it is on the same plane of existence, you can summon that weapon as a bonus action on your turn, causing it to teleport instantly to your hand.
You can have up to two bonded weapons, but can summon only one at a time with your bonus action. If you attempt to bond with a third weapon, you must break the bond with one of the other two.
However, if the weapon is a magic weapon that requires attunement, only one person can be attuned to it.
As for breaking someone else's bond with a weapon, this is probably only possible with a well-worded Wish. Dispel Magic is the go-to spell for getting rid of annoying magic, but it only works on ongoing spells, which the bond clearly isn't:
At 3rd level, you learn a ritual that creates a magical bond between yourself and one weapon. You perform the ritual over the course of 1 hour, which can be done during a short rest. The weapon must be within your reach throughout the ritual, at the conclusion of which you touch the weapon and forge the bond.
Similarly, the bond isn't a curse, so Remove Curse won't help. You can render the bond useless by being on a different plane of existence to the Eldritch Knight, or being in an Antimagic Field. On the bright side, depending on how your DM interprets the phrase "you can't be disarmed of that weapon", it might be impossible for the other Eldritch Knight to summon it while you're holding it.
The co-operating knights can take turns to use it, but again, depending on how your DM interprets the phrase "you can't be disarmed of that weapon", they might need to drop it at the end of their turns so that it can be summoned by the other knight. Even if they don't have to do this, they're still going to have problems making opportunity attacks, since only one of them will actually be holding a weapon at any given time.
Best Answer
RAW - No, Ballista is not a weapon; RAI - Unlikely
The Eldritch Knight Weapon Bond feature states:
Nowhere else in the description of the feature does it provide a restriction to what kind of weapon it can be. Only that you cannot have more than 2 of them.
What counts as a weapon?
The important qualification for your DM to decide on is what counts as a weapon. A strict RAW interpretation would be that only items which appear in the weapons table, or are listed as "simple/martial melee/ranged weapon" in their stat-block, count as weapons.
Another question; Can the Eldritch Knight bond with his shield? says that you cannot bond with a shield since it is not a weapon. Even though you could use it as an improvised weapon. Improvised weapons that are similar to actual weapon can be treated as that weapon and should qualify for your feature. Any others would not.
In this situation, how do we classify a siege weapon? On page 255 of the DMG Siege Equipment we are given the stats for siege weapons. From there we have:
Noticeable it is listed at an object despite being described as a siege weapon and having a ranged weapon attack. In contrast a heavy crossbow is listed in the PHB as a "martial ranged weapon". This means that by RAW you cannot use a ballista as your bonded weapon, as it is an object not a weapon.
For a RAI interpretation I think we get a clue from the next paragraph of the Weapon Bond feature:
From this I think it is clear that the RAI is that the weapon should be something you can hold in your hands. Large siege weapons probably don't fall into this category. If a ballista were small enough to fit in your hands it is basically just a heavy crossbow and I would rule it has the same stats.