How can I count rows that meet a condition inside each group?

In SQLite, group by the person identifier and put the condition inside the attended count. Assuming the table is named attendance and has columns person and status, use:

SELECT
  person,
  COUNT(*) AS total_sessions,
  COUNT(*) FILTER (WHERE status = 'attended') AS attended_sessions
FROM attendance
GROUP BY person
ORDER BY person;

COUNT(*) counts all rows in each group. The FILTER clause makes the other aggregate count only rows where its condition is true. [S1] The status text must match the values in your table; if yours uses a different label, replace 'attended' with that exact value. The aliases give the two results clear column names.

Assume these are the recorded rows:

person | status
Ari    | attended
Ari    | missed
Ari    | attended
Bo     | missed
Bo     | attended

The query returns:

person | total_sessions | attended_sessions
Ari    | 3              | 2
Bo     | 2              | 1

For Ari, the group has three rows, so the total is 3; two rows satisfy the attended condition, so that count is 2. Bo's group has two rows, and one satisfies the condition. These are illustrative values based only on the assumed rows. The query groups by person, so it calculates the two counts separately for each person rather than producing one count for the whole table.

This query groups the rows in attendance, so its results include people represented in those rows. A person with no attendance row will not have a group in that input to count. To include everyone, you would need to begin with a separate list of people and combine it with the attendance data; the exact query depends on your table names and keys. The supplied aggregate behavior establishes the counts within groups, but not the details of that separate-table setup. [S1] If you only need counts for people who have recorded sessions, the query as written is sufficient.

The FILTER form shown here is documented for SQLite; the available evidence does not establish equivalent syntax or version availability in other database products. [S1] Check the documentation for your database and version before changing the query. In particular, confirm how that system expresses a condition within an aggregate, then keep the same grouping and output goals: one total count per person and one count limited to attended rows. Do not assume the SQLite syntax will work unchanged elsewhere.