Invite registration, SQL statements do not know how to write,

Invite registration, SQL statements do not know how to write, Help!!!!
The database is MySQL and there are four fields in data table A, respectively, Id,name,password,pid

The ID is the auto-growing primary key.

PID is the ID of the inviter at the time of registration, default is 0

Four piece of data

ID name Password PID
1 Test 121121 0
2 test2 212121 1
3 Test3 415454 1
4 Test4 1212121 3

How can I use an SQL statement to show who invited whom? How many people did you invite?
------Solution--------------------
Not tested, adjust yourself or ask again

Select U.id, COUNT (ui.id), Group_concat (ui.id)
From Users U
Join Users UI
On U.id=ui.pid
GROUP BY U.id
;

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