HABTM-Relation : Create a relation for all other records

If i want to add a record to TABLE A, is there an efficient way to add
a record in the JOIN TABLE for many (or all) records in TABLE B?

I try to build a simple task management (in CakePHP). An user adds a task and there
will be added a connection to each other user in the same group as the
current user.

At the moment, I use the find('list')-method to retrieve the IDs and
store them in a variable. But I think if the groups grow, the PHP
cache won't handle this amount of data in a single variable.