Importing users – BuddyPress group assignment not working

When importing users from CSV file, the users are created as well as all their profile fields but they are not assigned to BuddyPress groups.
I am using the following fields:
Username
Email
last_name
display_name
user_nicename
bp_group (group slug)
bp_group_role (Member)
and some custom profile fields.
All works but the group membership is not assigned.