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I was just about to invite you...

I'd say "(마침) 초대하려던(or 초대드리려던) 참이었어요."


\^o^/

I was just about to invite you" to older brothers? "초대 드리려던 참이였죠" ?
-> Yes, Right. :)

-> Detailed..
-> '막 초대 드리려던 참이였죠' * '막 is translated to [just]

All answers are very appropriate..
I can attach somethings :
저는(I) 마침(or 막 : Just) 초대 드리려던(to invite) 참이었죠(about: but it is little complicated expression). And you should use '었죠' instead of '였죠'.But it is too hard spells even for natives XP