There are a number of "beta" agreements available on the web that you might consider as starting points (or templates to give to an attorney to craft into your own).
Additionally, consider what you want to get out of the Beta: If you simply want a name to use for marketing, then ensure that's in the agreement (obligation to support your marketing efforts). If you really want to get end-user feedback, either ask the company to put some skin in the game (money, committed to levels of use, etc...), and consider rewarding them in some capacity if they achieve your goals (x number of transactions, x number of users, y number of bugs, etc...). Rewards could be discounts on purchase when it exits Beta, participation on customer advisory boards, etc...
Additionally, consider what you want to get out of the Beta: If you simply want a name to use for marketing, then ensure that's in the agreement (obligation to support your marketing efforts). If you really want to get end-user feedback, either ask the company to put some skin in the game (money, committed to levels of use, etc...), and consider rewarding them in some capacity if they achieve your goals (x number of transactions, x number of users, y number of bugs, etc...). Rewards could be discounts on purchase when it exits Beta, participation on customer advisory boards, etc...