Although "It's I" is technically correct, it's considered pedantic (excessively correct). It's OK to use "It's me." IT's also OK to say "It's us."
With other personal pronouns you can use either the subject form or the object form. The subject form isn't considered pedantic, but it's more formal. Google examples:
"”Further, it was they who took the first steps towards civilization, ... It was they who had nothing to begin with but eventually discovered almost ...
"”It wasn't they who conquered and looted entire continents for centuries, nor did they establish colonialism, nor did they reintroduce slavery, ...www.commondreams.org/headlines02/0322-05.htm
Much more informally, the object form is used:
At the time Alda was working as a secretary for the Japanese Ambassador and it was him who helped them come to a decision. "The ambassador said to me ˜why ...www.bbc.co.uk/kent/content/articles/
"”"It was them who inspired me to become a bird watcher," he adds. Even in the recent trip, Halder, a dentist by profession and an amateur ornithologist, ...www.thedailystar.net/magazine/2005/01/02/focus.htm
"”Because of course, it wasn't she who made the schedules of daily things, and it wasn't she trained the soldiers standing guard over her even now, nor was ...