Hi Mark,
Yes, you can host other users on your Office 365 plan. It will be more easily to administrate the accounts that subscribing a separate Office 365 plan.
If the users want to use their own domain, you need to verify the domain on the Office 365 plan.
Add your domain to Office 365
http://onlinehelp.microsoft.com/en-us/office365-enterprises/ff637620.aspx
Thanks,
Aaron Yuan
Yes, you can host other users on your Office 365 plan. It will be more easily to administrate the accounts that subscribing a separate Office 365 plan.
If the users want to use their own domain, you need to verify the domain on the Office 365 plan.
Add your domain to Office 365
http://onlinehelp.microsoft.com/en-us/office365-enterprises/ff637620.aspx
Thanks,
Aaron Yuan