self join with inner join on other tables together

hii how can i make self join and sum data in other table by username
i have table like this

members (id, username, referral)
ref_bonus (id, username, bonus)

members
id  | username | referral
----------------------
1    id1        id2
2    id2        null
3    id3        id2

ref_bonus
id | username | bonus
----------------------
11    id2      1.00
21    id2      1.00
31    id2      1.00

the result i want is select all member have referral and sum bonus in ref_bonus
example result is like this :

id  | username | count_ref | sum_bonus
----------------------
1    d2          2         | 3.0