From #openttdcoop wiki
Since there a some people around who can't use a voicecome due to several reasons, it is highly necessary to have an improved team-chat within OTTD itself.
Currently, if you type <enter> and write a message, you write it to the whole server which means: every other player including spectators may see your message. Only via the userlist you may select and write a team or user message. Therefore, you currently need to use your mouse.
Suggestion for a teamchat:
- Activate it with a keyboard-command like ctrl+<enter>
- Even better: create two buttons: @all-mode and @team-mode. If a button is activated, the chat STAYS in the selected mode.
JohnK and I (Osai) created a final Solution for this problem
Download the file here: http://openttdcoop.ppcis.org/blog/files/patches/talk_to_team_final.patch