…is to write about the people you want in it.
If you want managers of London advertising agencies to participate in your online community, then create a community just for them. Write about what managers at London advertising agencies are doing.
You might write what advertising campaigns they’ve just launched, interview one a week, write rumours and gossip about managers at London advertising agencies, rank them and have a manager of the month.
Very few people can ignore a community about them and their peer group. Keep it specific to content about managers at London advertising agencies.