select t1.*, t2.*
from t1 left join t2
on t1.id=t2.t1_id
order by t1.id, case when t2.t1_id is null then 1 else 0 end, ...

mysql> create table t1(id int, event text);
Query OK, 0 rows affected (0.05 sec)

mysql> insert into t1(id,event) values (1,'Event 1'), (2,'Event 2'), (3,'Event 3'), (4,'Event 4'), (6,'Event 6'), (7,'Event 7');
Query OK, 6 rows affected (0.00 sec)
Records: 6  Duplicates: 0  Warnings: 0

mysql> create table t2(t1_id int, user_id int);
Query OK, 0 rows affected (0.03 sec)

mysql> insert into t2(t1_id, user_id) values (4,37),(7,37);
Query OK, 2 rows affected (0.00 sec)
Records: 2  Duplicates: 0  Warnings: 0

mysql> select t1.*
    -> from t1 left join t2
    -> on t1.id=t2.t1_id
    -> order by case when t2.t1_id is null then 1 else 0 end;
+------+---------+
| id   | event   |
+------+---------+
|    4 | Event 4 |
|    7 | Event 7 |
|    1 | Event 1 |
|    2 | Event 2 |
|    3 | Event 3 |
|    6 | Event 6 |
+------+---------+
6 rows in set (0.00 sec)