Creating a LOGIN requires server-level permissions, so it doesn't really matter what permissions you grant within the database, even a user with dbo/db_owner permission won't be able to create a login.

I'd be inclined to sign the procedure to grant it rights to create a login. You can read some about signing a procedure in these places:

http://sommarskog.se/grantperm.html

http://msdn.microsoft.com/en-us/library/bb283630.aspx