BuddyPress.org

>Buddypress is waiting for the first login of each user

At which point it will add a last_activity timestamp.
The easiest solution is to add those timestamps for everyone via a script.
Put this code in your bp-custom.php file, then open your site in a browser.
Then remove the code – or it will update the timestamp on every page load.

function daten_add_last_activity() {
    global $wpdb;
    foreach ( $wpdb->get_col( "SELECT ID FROM $wpdb->users" ) as $user_id ) {
        bp_update_user_last_activity( $user_id, bp_core_current_time() );
    }
}
add_action('bp_init', 'daten_add_last_activity' );

Beauty of forums are their archives, the question has arisen a few times:
https://buddypress.org/support/topic/how-to-activate-existing-wp-users-as-bp-members-effectively/

I needed a solution myself a few years back to activate imported users so ran a function up and uploaded to gist:
https://github.com/hnla/force-show-bp-users

Just updated it to reflect BP 2.0 changes to last activity to use ‘bp_update_user_last_activity()’ – it takes a slightly different approach in running only as admin and requiring that user runs the script by passing a get variable on the url ?action=force-active just to ensure script wasn’t run by accident or repeatedly also it checks and runs on a user id only if that user has no existing entry as I had existing users, I also didn’t want them all to have same time stamp so added in a delay on each iteration of the loop so each had a unique timestamp and just to allow breathing space between updates if loop particularly long.

Updates to script are untested at this time.