If a user with no special privileges is made a member of a closed group (but not an owner) and then browses to

/co_groups/select/co:N

the row in the generated form corresponding to that group will have HTML in the "Actions" column (for selecting "Member" and "Owner") like this:

<td class="actions">
<input type="hidden" name="data[CoGroupMember][rows][3][co_group_id]" value="725" id="CoGroupMemberRows3CoGroupId">
<input type="hidden" name="data[CoGroupMember][rows][3][id]" value="13930" id="CoGroupMemberRows3Id">
<input type="hidden" name="data[CoGroupMember][rows][3][member]" id="CoGroupMemberRows3Member_" value="0" disabled="disabled"><input type="checkbox" name="data[CoGroupMember][rows][3][member]" disabled="disabled" checked="checked" value="1" id="CoGroupMemberRows3Member"><label for="CoGroupMemberRows3Member">Member</label><input type="hidden" name="data[CoGroupMember][rows][3][owner]" id="CoGroupMemberRows3Owner_" value="0" disabled="disabled"><input type="checkbox" name="data[CoGroupMember][rows][3][owner]" disabled="disabled" value="1" id="CoGroupMemberRows3Owner"><label for="CoGroupMemberRows3Owner">Owner</label> </td>

The checkbox for "Member" has

disabled="disabled"
checked="checked"
value="1"

but the associated hidden input has

disabled="disabled"
value="0"

So if the user just clicks "SAVE" the form input includes the value="0" and the user is removed from the closed group.