New to the game, how do I join a player attacking Wendell?

You can join a rally started by another alliance member during the rally timer. You need to be on the same alliance and in the same state (occasionally you may be in a different state for an event)

Open the alliance menu (expand the right lower menu and tap the star emblem) and select the 'War' option. This shows any active rallies or attacks. Tap on the rally you want to join and if there are available spaces tap on one to then select an APC to send to join the rally. You must be able to march to the Rally leaders base in time before the rally prep timer ends. Rallies need at least 3 participants to start.

If you see a notification in the alliance chat about a rally then you can also tap on that to go directly to the rally screen.