Joining mid-round
Joined while a round is running? You watch until the next map, unless the host lets you in.
You watch first
If you join a room while a round is running, you spectate until the next map starts, then you spawn in with everyone else.
The host can let you in
The host's pause menu has a "Waiting for next map" list with a "Let in" button for each person. Press it and that player spawns right away. When nobody is waiting, the list says so.
Only the host sees this list. Everyone else just waits for the next map.
On the scoreboard
Players who are waiting are marked as "Joining next map" on the scoreboard. While they wait, they can vote on ghost cards like any other spectator, as long as ghost votes are on.