Log into Steam > Settings > Family > Authorize Library Sharing on this device.

You need to be friends first. You will see the eligible accounts under Settings > Family.

Yes, a Family Library can be shared with up to 5 accounts and up to 10 devices in 90 days.

Yes, you can share a specific game by flagging all the other games as private.

Offline and solo games are 100% safe, but you can get a VAC ban if your copy is used to cheat in a multiplayer game protected by Valve Anti-Cheat (VAC). Only the cheater and the owner of the copy will be affected by the ban.