Green’s Dictionary of Slang

1. (US campus) an expression of support.

1893		P.H. Emerson Signor Lippo 9: ‘Dry to-day, mates?’ ‘Yes, dry as a sandbank; got any lush?’ he said. ‘I’m there,’ says I, ‘come along,’ and they followed quick enough and I called for porter.
1909		H. Green Mr. Jackson 141: Oh, I’m there with the spiel, all right.

2. an expression of enthusiasm, lit. a promise to attend.

1977		Eble Campus Sl. Apr. 2: I’m there – an expression of enthusiastic approval: A. There’s a big South Campus bash on Friday night. B. I’m there!
1981		Eble Campus Sl. Fall 7: we’re there – enthusiastic commitment to be somewhere.
1992		D. Burke Street Talk 2 24: I just found out that my favorite music group is performing at the amphitheater! I’m there!