Limit Users registration based on the site level

Hey, guys am a newest kid on the block. lol just a quick question/thought. is there a plugin that can use along with the Pro Site plugin, such that it can add a limitation as it relates to "site users registration". Ex: Pro sites uses different levels, where a site admin can have access to numerous plugins , features etc.. is there a plugin that can be use along with the user level section of Pro site that can limit the amount of user X site can have, based on the site level. Hope this is clear enough. Thanks for the replies.

By the way i've just purchase pro site and haven't fully try it yet.