Do I need to login/maintain an account on these other machines?
No, just once to activate Steam Guard on their machine
If someone is playing a shared game do I get locked out of my game library until they are done?
No, they will be kicked out of their game to allow you to play. Per Steam Family Sharing FAQ:
When I authorize a device to lend my library to others, do I limit my own ability to access and play my games?
As the account holder, you may always access and play your games at any time. If you decide to start playing when another user is already playing one of your games, he/she will be given a few minutes to either purchase the game or quit playing.
Clarifying for @WernerCD - any game activated on Steam will kick the person with whom the library is shared.
If they do something stupid like obtain a VAC ban am I subject to the same punishment?
Yes, your account might get banned for any cheating/fraud conducted by the one you share your library to. Per Steam Family Sharing FAQ:
Will I be punished for any cheating or fraud conducted by other users while playing my games?
Your Family Library Sharing privileges may be revoked and your account may also be VAC banned if your library is used by others to conduct cheating or fraud. Additionally, VAC-banned games cannot be shared. We recommend you only authorize familiar computers you know to be secure. And as always, never give your password to anyone.
Are there any (other?) disadvantages to myself if I share my games?
Not really, only disadvantages are on the other side (limited availability when the library owner is playing, no offline mode to play shared games, no trading cards gathering, etc.)
Steam Family Sharing FAQ can be found here.
To an extent, yes and no.
As the Steam Family Sharing FAQ says,
Can I share specific games, or do I have to share my whole library?
Libraries are shared and borrowed in their entirety.
That's the "no" part. However technically there is a way to enable only selected games for accounts you're sharing with. You can do this by using Family Options. Here's how.
First, login to the account you want to be used (the account you want the game[s] to be shared with) to unlock the library.
Secondly, then with that account logged in, activate the Family Options with a set PIN.
Then whenever you want to set a suitable game for that person, log yourself into their Steam client, unlock Family Mode with the PIN, and set the new game to " Family Games" and then lock the account again.
Best Answer
No, there isn't. The most that is possible inside the game is the scan for nearby discoveries, and that usually only works when your friends are quite close by. Currently, Sean has said on his twitter here and here that multiplayer is included, but the servers are too overloaded to cope with it at the moment. This leads us to believe that it may be a while before "your friend showing up on the galactic map" is in the game. So that option isn't impossible, but highly unlikely at the time of writing.
Outside the game, Sean and Hello Games haven't made anything that links to the inside of the game, like an API, just yet. The community itself is working on some things, such as a list of all crowdsourced discoveries and more. However, apart from these and simple things such as recording and screenshots, there is no way to share your discoveries with friends.