In Act V, Malcolm and Macduff bring their army to attack Macbeth and his followers at Dunsinane. In Act V, Scene 2, Angus and Caithness indicate that Macbeth is beginning to lose the loyalty of his men: "Those he commands move only in command, / Nothing in love." (V.ii.22-23).
Meanwhile, Malcolm's army marches on. Dressed with branches to camouflage themselves, Malcolm hopes to fool Macbeth as to how large his army actually is:
Let every soldier hew him down a bough,
And bear't before him: thereby shall we shadow
The numbers of our host, and make discovery
Err in report of us. (V.iv.6-9)
So, at this point, Macbeth still thinks he will not be killed by "one of woman born" but he realizes that Birnam Wood has come to Dunsinane and that he's outnumbered. This is a result of the camouflage and the fact that some of his men have switched to fight on Malcolm's side. This is what Malcolm means when he says that he has met with foes (some of Macbeth's men) who have decided to fight with him (Malcolm) instead. "Strike beside us" means to "fight with us."