。
I'm thinking of inviting some friends over.
Literal
Friends [object-を] call-[volitional-quotative-ようと] think-[ongoing-ている]-[explanatory-んだ].
~ようと思っている ('thinking of doing X') is a common pattern for expressing tentative plans. The ~よう here is the casual volitional form (呼ぶ → 呼ぼう), plus と (quotative) plus 思っている ('is thinking'). Literally 'I am in the state of thinking, let me call friends.' The んだ framing adds an explanatory feel.