How can I save the result of a query in an array using MySQL stored procedure?

Although MySQL doesn't have a data type ARRAY, I think what you want is to use the result of your query later, without having to write it over and over again. This is possible: you can save the result in a temporary table, and use it as many times as you want:

-- Cambié el JOIN implícito (tu condición WHERE) por un JOIN explícito, por claridad
create temporary table temp_resultado
    SELECT tabla1.campo1 
    FROM tabla1 INNER JOIN tabla2 ON tabla1.campo1 = tabla2.campo1
    GROUP BY tabla1.campo1;
-- No olvides generar un índice:
ALTER TABLE temp_resultado
    ADD INDEX idx_campo1(campo1);

Now, if what you need is to assign a number to each record, you can use user variables:

create temporary table temp_resultado
    SELECT @n := @n + 1 as n
         , tabla1.campo1 
    FROM (select @n := 0) as init, (tabla1 INNER JOIN tabla2 ON tabla1.campo1 = tabla2.campo1)
    GROUP BY tabla1.campo1;
-- No olvides generar un índice:
ALTER TABLE temp_resultado
    ADD INDEX idx_campo1(campo1);

Remember: Temporary tables are only visible to the connection that created them, and are destroyed once the connection that created them is closed.

update

As you can see, I am not using a stored procedure to generate the table you require, but rather I am generating the row numbers you need "on the fly".

How does it work?

SELECT @n := @n + 1 as n
     , tabla1.campo1 
FROM (select @n := 0) as init
   , (tabla1 INNER JOIN tabla2 ON tabla1.campo1 = tabla2.campo1)
GROUP BY tabla1.campo1;

In the clause FROMI include your two tables, and a subquery (which I call init) to initialize the variable @n. Remember that in SQL the "order of execution" is:

1. FROM: The server checks that the data sources exist and can be read; In addition, the required relationships are established
2. WHERE: The relevant filters are applied to the data sources
3. GROUP BY: The data is grouped
4. SELECT: Fields are read and expressions are calculated (including aggregation expressions
5. HAVING: Relevant filters are applied to the data after it has been grouped

So, taking advantage of the fact that the first thing that is executed is the FROM, I put a variable initialization query ( init) there. The variable @nis initialized to zero, and each time it SELECTreads a row, it updates the value of @n, adding 1 to it.

Finally, by creating a temporary table, you guarantee that the row numbers aren't updated every time you run the query (since you're not ordering the values, there's no guarantee that an ordering of the rows will be respected).

Once you run the above queries, you can use these values ​​simply by selecting all rows from the newly created table:

select * from temp_resultado order by n;

Now I see you're not using any aggregation functions; I think it's easier then to use SELECT DISTINCT:

drop table if exists temp_resultado; -- Si existe la tabla, bórrala
create temporary table temp_resultado
    select distinct @n := @n + 1 as n, tabla1.campo1
    from (select @n := 0) as init
       , (tabla1 inner join tabla2 on tabla1.campo1 = tabla2.campo1)
       -- Si el campo que usas para relacionar las tablas tiene el mismo nombre,
       -- puedes usar:
       -- (tabla1 inner join tabla2 using (campo1))
    order by tabla1.campo1; -- Generalmente es prudente especificar 
                            -- el orden de los registros
-- No olvides agregar los índices:
alter table temp_resultado
    add unique index idx_n(n),
    add index idx_campo1(campo1);
-- Muestra el resultado
select * from temp_resultado;

Why is "on the fly" better than with a procedure?

If you want to try a stored procedure, consider the following:

1. You need a table to store the data
2. You need to insert the data into that table
3. You need to iterate through that table row by row to add the value you want (although you could use a value AUTO_INCREMENTin the table definition, you'd want the procedure to store the number instead)

For mere economy of keys, you should generate the values ​​"on the fly". On the other hand, by generating the values ​​"on the fly" using temporary variables, you're not really giving the server too much extra work.