Absolutely.

Say "Absolutely" to express that you completely agree with what someone has said.

You can agree with someone's opinion this way.

A: I've always said that you shouldn't hire anyone who you don't feel 100% certain about.

B: Absolutely. I totally agree.

You can also agree to a request:

A: Hey, can I talk to you for a minute?

B: Absolutely.

"Absolutely" sounds even more definite than "yes". You answer "Absolutely" when you're really sure that the answer is "yes".

count (me/us) in

When someone invites you to participate, you can say "yes" with the phrase "count me in":

A: I feel really good about this business opportunity so I was hoping that you'd be interested in investing.

B: Yeah, count me in.

Or you can speak for yourself and someone else, like your partner or friends:

If you guys ever decide to organize a camping trip, definitely count us in.