Collapsable, grouped field in View (Bootstrap, D8)
I solved it using Bootstrap. I hid the field, then added it one more time. Then I rewrote the second field like this:

&lt;a href="#{{ nid }}" class="btn btn-info" data-toggle="collapse"&gt;
Show members&lt;/a&gt;
&lt;div id="{{ nid }}" class="collapse"&gt;
{{ field_research_group_ext_members }}
{{ field_connected_user_names }}
&lt;/div&gt;