How do you play this online?

[BananaJane]

I can't join my friend's server

[Snoopy]

you may need to use the console to enter the connect command along with your friends IP address of the server. In the console type: connect <IP>:<PORT>
and of course press enter to execute the command.
<IP> should be replaced with the actual IP address of the server
<PORT> with the port used for the server
and don't forget to put a : between the two (ip:port) with NO spaces

[Spawndex]

If whoever is running the server has forwarded port 27015 and you can't join their server by typing in connect and their IP into console then you could always try downloading a programme called Tunngle, that worked for me when I wanted to play with a friend and couldn't get it working

its got really annoying obnoxious adverts all over it but it does tend to work when other methods fail, you just have to use the network search bar to search for synergy and both of you join it together and then when they launch the game whilst in the tunngle synergy lobby with you you should be able to see their server in the LAN tab on the synergy server browser

good luck