This is a smart move. You both play a motivation game in which you see each other's results. And keep each other honest. You can promise to do the same thing (e.g. exercise 4 times a week) or create completely individual promises.
There's space below to use a code word, to reassure the person you're inviting that the invitation is not spam.
Totally optional: fill in the last two boxes to let your buddy know the actions you plan to stake.