Update post meta dynamically

Basically, you’re not properly using array_push().

array_push() modifies the original/input array which is passed by reference to the function and it returns the new number of elements in the array.

So with add_post_meta( $groupItem->ID, $meta, array_push( $user_ids, 26 ) ), you’re actually setting the meta value to the number of items in $user_ids and not the items in the array — so for example, add_post_meta() would get a 1 instead of a [26].

So if you want to use array_push(), you could do it like so:

if ( ! $user_ids ) {
    $user_ids = [];
    array_push( $user_ids, 26 );
    add_post_meta( $groupItem->ID, $meta, $user_ids );
} else {
    $prev_user_ids = $user_ids; // backup old values
    array_push( $user_ids, 26 );
    update_post_meta( $groupItem->ID, $meta, $user_ids, $prev_user_ids );
}

Or simply use $user_ids[] = 26; ..