var client = new MongoClient("mongodb://localhost:27017");
var database = client.GetServer().GetDatabase("mydb");
var users = database.GetCollection<IdentityUser>("users");

var context = new IdentityContext(users);
var store = new UserStore<IdentityUser>(context);
var manager = new UserManager<IdentityUser>(store);

// if you want roles too:
var roles = database.GetCollection<IdentityRole>("roles");
var context = new IdentityContext(users, roles);

// at some point in application startup it would be good to ensure unique indexes on user and role names exist, these used to be a part of IdentityContext, but that caused issues for people that didn't want the indexes created at the time the IdentityContext is created. They're now just part of the static IndexChecks:

IndexChecks.EnsureUniqueIndexOnUserName(users);
IndexChecks.EnsureUniqueIndexOnEmail(users);

IndexChecks.EnsureUniqueIndexOnRoleName(roles);