This is pretty common for most of the users but good info for newcomers. Basically there are few different ways to join the server as a player or spectator:

The first way:
- You can control whether or not you are a spectator by client command /spectator 1 or 0.
- After this you can use /connect ip:port command (or reconnect if you are on the server).

The second way:
- You can use /join ip:port or /observe ip:port whether or not you are connected to a server.

The third way:
- Once you are on the server, you can change your spectator/player status by just typin /observe, /join, or /togglespec.

The fourth way:
- Use the inbuilt server browser (keys there are J for join and O for observe).