ISIS terrorist events lead to short term spikes in people in the UK searching Google using the words "join" and "isis". The relative scale of the impact of different events on the volume of people searching Google for how to join ISIS can be seen below.
Data is taken from Google Trends measuring searches which include both the words "join" and "isis", but excluding the word "against". This will count searches such as "join isis" and "how to join up with isis", but not searches such as "how to join fight against isis".
Of course many people who want to join ISIS may take a more sophisticated approach than Googling "How do I join ISIS?", and many people who do Google such terms may also not be making a serious consideration for joining ISIS but may be searching from a purely speculative perspective.
Events of greater magnitude and drama have tended to cause more people to search Google for these search terms, although recently the Barcelona, London, and Manchester attacks have caused relatively small spikes.
There is undoubtedly a strong correlation here between significant ISIS terrorist events and people Googling for terms like "How to join ISIS", and it is logical that a dramatic and effective event aligned with a rationale may lead to an increase in people who have an appreciation for the rationale to take the next step in trying to join the organisation that claimed responsibility for the event. However, given the data here it would be very much a matter of subjective judgement to estimate the proportion of these people who are Googling and seriously considering joining ISIS, as opposed to those that are Googling speculatively or for other reasons.