Everyone who donates $50+ is added to the "Members+" group on the XBMC forum to indicate that you've shown support for XBMC and the XBMC Foundation. Please provide your forum nickname in the "Note to seller" field when you submit the donation so that you may be identified.
If you are not listed as being part of Members+ on the forum a week after donating (PayPal receipts are not always delivered quickly), please send a mail with your username and donation date/amount (plus your paypal receipt) to email@example.com and we'll fix it up.
Joining "Members+" gets you no additional features, nor any early access to beta features. XBMC operates entirely in the open. The moment a feature is coded, it is available to anybody who can compile it. If you join Members+, you are doing so purely to show support for the software and the non-profit Foundation behind it.
You can become a member of Members+ by either donating $50 in a single lump sum, or by donating smaller amounts over time. If you cross the $50 threshold with smaller amounts, you will need to email firstname.lastname@example.org with the email address you used to donate and your forum name.