How to Select a column in the inner join without group by SQL

I have three tables users -> user_challenge <- challenge
and has many to many relationships

users table :

id | name | user_email
---+------+------
777 | "abc" | abc@abc.com
888 | "bcd" | bcd@bcd.com

user_challenge table

id | user_id | challenge_id
---+---------+-------------
1  |  777   |  22
2  |  777   |  23
3  |  888   |  23

challenge table

id | title | user_id
---+---------+-------------
21  |  "go"   |  777
22  |  "run"  |  888
23  |  "eat"  |  888

I want to group the number of users who is in the challenge table ( works fine here )

SELECT c.Id, c.title, COUNT(*) AS Count_joined
FROM Challenges AS c
   JOIN
   user_challenge AS ug
   ON c.id = ug.challenge_id
    JOIN
    users AS u
    ON ug.user_id = u.id

GROUP BY c.Id, c.title;

but when I want to take their email, so I added the email column in the users table like this :

SELECT c.Id, c.title, COUNT(*) AS Count_joined ,  u.email
FROM challenges AS c
   JOIN
   user_challenge AS ug
   ON c.id = ug.challenge_id
    JOIN
    users AS u
    ON ug.user_id = u.id

GROUP BY c.Id, c.title;

It shows error [S0001][8120] Line 1: Column 'users.email' is invalid in the select list because it is not contained in either an aggregate function or the GROUP BY clause.

I need the result same as the following :

id | title | user_email
---+---------+-------------
21  |  "go"   |  abc@abc.com
22  |  "run"  |  bcd@bcd.com
23  |  "eat"  |  bcd@bcd.com

How I can select the email column for the same result I got from the first piece of the statement?

SELECT
  c.id,
  c.title,
  u.user_email,
  COUNT(uc.id) AS participant_count
FROM
  challenges        AS c
INNER JOIN
  users             AS u
    ON c.user_id = u.id 
LEFT OUTER JOIN
  user_challenge    AS uc
    ON c.id = uc.challenge_id
GROUP BY
  c.id,
  c.title,
  u.user_email

You just need to add email column in your group by clause

GROUP BY c.Id, c.title, u.email;