Link Active directory with Office 365 Midsize Business Single plan

Hi,

1)I am planning to use Office 365 Midsize Business to create a social site. I would like to know how many normal users can be create using a single license copy by paying 900/-. 

2)Will I be able to link my active directory with Office 365 Midsize Business Single Plan and give permission to all active directory users to access office 365(the social site which I am creating). 

Thanks