Techniques include: receiving a first search query from a user; determining a topic associated with the first search query; sending a second search query to the user, where the second search query is associated with the topic and is different from the first search query; receiving a selection of the second search query from the user; and, when the second search query has been received from other users more than a specified number of times within a time window, sending an invitation to the user. The invitation may be for inviting participation in an online social group that includes others who have sent the second query to the server system within the time window. The invitation can be for inviting participation in an online social group including other users (e.g., members of a social network) who have sent the second query to the server system within the time window.