The problem I was having was that the activity ID wasn't being received by the controller method. In params, I had [:activity] and not [:user]. Basically, the form and controller are adding a record to the join table for activities_users, submitting an ID for both activity_id and user_id. I got the help of my roommate, who is a programmer, and n…

edit.html.erb

<h2>Favorite Activities</h2>
  <ul>
		<% User.find(current_user.id).activities.each do |a| %>
			<li><%= a.name %> 
				<%= form_for @user, :url => removefavorite_path do |f| %>
					<%= f.hidden_field :activity_id, :value => a.id %>
					<%= f.submit('Remove') %>
				<% end %>
			</li> 
		<% end %>
	</ul>

users_controller.rb

  def addFavorite
    Activity.find(params[:user][:activity_id]).users << current_user
    redirect_to :back
  end