Add, remove, and append entries to an entries field via frontend form
You guessed right, you have to set all already added entries again. To do this,
just have some Twig logic when outputting the fields:

{% for currentTeam in entry.tournamentTeams %}
    &lt;input type="hidden" name="fields[tournamentTeams][]" value="{{ currentTeam.id }}"&gt;
{% endfor %}
&lt;input type="hidden" name="fields[tournamentTeams][]" value="{{ team.id }}"&gt;


To remove the entry:

{% for currentTeam in entry.tournamentTeams if currentTeam.id != team.id %}
    &lt;input type="hidden" name="fields[tournamentTeams][]" value="{{ currentTeam.id }}"&gt;
{% endfor %}


Update: What I'm not sure about is if the above code works when trying to remove the last remaining entry. If it doesn't work I'd try and add something like this:

{% if (entry.tournamentTeams|length == 1) and (entry.tournamentTeams.first().id == team.id) %}
    &lt;input type="hidden" name="fields[tournamentTeams]" value=""&gt;
{% endif %}