You can pass array of addresses as well:
def welcome_email(users)
@url = 'http://www.fixrnix.in'
mail(to: users.pluck[:email] , subject: 'Welcome to FixNix Audit Management')
end
However, if your email body depends on the user or you don't want mailed users to see other users's addresses you can't do this. You need to create a new message for each user and send it separately.
users.each do |user|
YourMailer.welcome_email(user).deliver
end