creating a join action between user and group correctly

The reason your code isn't working now is because of this line:

@group = Group.find_by_name(:group)

it should be something like (I don't remember exactly sorry)

@group = Group.find_by_name(params[:membership][:group])

The error is getting called on the next line because @group is nil.

But you should probably handle that type of logic in the model anyway with a virtual attribute or something.

membership.rb

def group_name
  if self.group
    @group_name ||= self.group.name
  end
end

def group_name=(group_name)
  @group_name = group_name

  self.group = Group.find_by_name(@group_name)
end

form

<div class="field">
  <%= f.label :group_name, "Group" %><br />
  <%= f.text_field :group_name %>
</div>

controller

def create
  @membership = current_user.memberships.build(params[:membership])
  if @membership.save
    flash[:notice] = "You have joined this group."
    redirect_to :back
  else
    flash[:error] = "Unable to join."
    redirect_to :back
  end
end