You can associate BuddyPress Member types with WordPress roles from the Dashboard->Settings->Member Types menu as shown below.

You will see the options as below to create a mapping from WordPress roles to Member Types

When you map a role to one or more member types, changing the role of a user will change their member types.  If you do not associate any member type to a role, changing role of a user will not affect the member type of the user.

For multisite, Please see the details here.