MSP comes with administrator installations for 4 computers. (Please note this is based on the number of computers, not number of people. Someone with two computers counts as two installations.) Additional computer installations can be purchased for a one time fee of $15 plus an additional $6 per month.
If you have the Plus Edition, you can allow people to access the program as Ministry Leaders. These Ministry Leaders have different privileges than an administrator, but can do some manual scheduling and send emails to volunteers.
For more information on the difference between administrators and Ministry Leaders, see this article: