There's nothing wrong with your form but it can be simplified to the following
<%= form_for @remark, :url => salveaza_responsabili_remark_path(@remark) do |f| %>
<% @users.each do |user| %>
<%= check_box_tag 'user_ids[]', user.id, @remark.users.include?(user) %>
<%= user.name %>
<% end %>
<% end %>
Then in your controller, you can expect an array from params[:user_ids]
def save_users
@remark = Remark.find(params[:id])
# This is where you need to think about things. If the checkbox in the form
# contains all the users for a remark, the following code should work.
#
# @remark.user_ids = params[:user_ids]
# @remark.save
#
# otherwise, you have to loop through each user_id
params[:user_ids].each do |user_id|
@remark.remark_users.create!(user_id: user_id)
end
end