。
You have to invite him, you know.
Literal
Him [object-を] not-inviting [must-なくちゃだめ] [assertion-よ].
Casual feminine speech. 誘わなくちゃだめ is a contracted chain: 誘わなくてはだめ → 誘わなくちゃだめ, meaning 'must invite.' The contraction ~なくちゃ is casual spoken Japanese for ~なくては. だめ ('no good') completes the obligation. The sentence-final よ adds feminine assertion — the speaker is telling someone what they need to do.