+----+----------+-------+------------+
| id | name     | admin | timestamp  |
+----+----------+-------+------------+
|  1 | Group1   |     2 | 1539678214 |
|  2 | Test     |     2 | 1539678214 |
+----+----------+-------+------------+

+----+----------------+------------------------+
| id | name           | email                  |
+----+----------------+------------------------+
|  1 | FOO            | foo@bar.com            |
|  2 | BAR            | bar@foo.com            |
|  3 | G2W            | far@boo.com            |
+----+----------------+------------------------+

SELECT projects.id,projects.name,users.id as userid,users.email FROM projects INNER JOIN users ON users.id=projects.admin;

+----+----------+--------+-----------------+
| id | name     | userid | email           |
+----+----------+--------+-----------------+
|  1 | Group1   |      2 | bar@foo.com     |
|  2 | Test     |      2 | bar@foo.com     |
+----+----------+--------+-----------------+

$stmt = $_DB->prepare("SELECT projects.id,projects.name,users.* FROM projects INNER JOIN users ON users.id=projects.admin");
if($$_DB->error) die($$_DB->error);

$stmt->execute();
$res=$stmt->get_result();
while($result[]=$res->fetch_array(MYSQLI_ASSOC)){}
print_r($result);

[0] => Array
    (
        [id] => 1
        [name] => Group1
        [userid] => 2
        [email] => bar@foo.com
    )

[1] => Array
    (
        [id] => 2
        [name] => Test
        [userid] => 2
        [email] => bar@foo.com
    )

[0] => Array
    (
        [id] => 1
        [name] => Group1
        [admin] => Array
            (
                [id] => 2
                [email] => bar@foo.com
            )

    )

[1] => Array
    (
        [id] => 2
        [name] => Test
        [admin] => Array
            (
                [id] => 2
                [email] => bar@foo.com
            )

    )