If you're having problems joining a friend's game in 'Play friends' mode, here are a few tips:
(1) Make sure you're both connected as friends
For more information on how to become RISK friends within the game, read this FAQ article.
(2) Join using a Game ID
You can join a friend's game by entering the game's ID. The Game ID is shown at the bottom of the host's lobby screen only (see below).
To join a game using a game ID given to you by the host, tap the 'Play friends' button on the home screen, then tap the 'Enter ID' button (shown below).
(3) Join with automatic detection over local wifi
Players can enable "Find local games" in the Settings menu to automatically detect RISK games on their local network. This feature is only available to players aged 13+.
(4) Make sure you and your friend are up-to-date
Make sure you both have the most recent version of the RISK mobile app installed. The app version number is found at the top left of the 'Settings' screen (shown below). Occasionally you may see that your friend is on a slightly different version 2.6.1 or 2.6.2 - this may just be due to platform-specific updates, and should generally work in conjunction with other 2.6.x versions, but not 2.7