BuddyPress.org

Using phpmyadmin, you should see 3 db tables.
Assuming your $table_prefix is wp_, you should see:
wp_bp_groups
wp_bp_groups_groupsmeta
wp_bp_groups_members

Make backups of all 3 tables.
Then select them, scroll down and select ‘Empty’ from the ‘With Selected’ dropdown.
That will delete all the data but leave the tables and their structure.