I share the internet with three, sometimes four, other people. If I use Tor, am I safe, or do the other people in my house compromise my Tor usage/security/etc.?
Take a look at this diagram by the EFF. It explains who can know what about you when you are using Tor.
Yes, it is safe to use Tor if you're on the same network with other people. In fact, Tor is one of the best tools to use when you're using a public network/wifi (i.e in your favorite coffee-shop).
And no, it doesn't compromise your Tor usage and security. As people have already explained, your connection to the Tor network is always encrypted with three layers of encryption (Just like an onion).
You might enjoy watching this short Tor animation, explaining how Tor works in simple terms:
Yes, it's encrypted from your machine's connection to the different nodes in the network, they might know you are using Tor if they start analyzing the local network data\connections, but they can't sniff the data.
It should not compromise you in any way.