Hello!
On the app version:
- Messages (bottom right of your city)
- Create message tab at the top of the window
- Add recipients button
- Choose an option from the drop-down box (Friends, Guildmates, etc). Then choose people from the list
- Press the Add Selected button, enter the message and send
On the browser version:
- Envelope icon on the bottom left of your city
- Create Message tab at the top of the window
- Type player name in the search box, suggestions will appear
- Click people to add to the message
- Enter your message, then press send
Please be courteous when creating group messages. Spamming neighbours over and over with unwanted messages, such as trade offer spam for example, is not only annoying but could also get you reported for spam.
Group messages are of course necessary for guilds, for example for FP swap threads and so forth.