A great opportunity to learn about recommender systems and build your network are summer schools. Unfortunately, there are only few summer schools on recommender systems and rather infrequently (If you know of other recommender-system summer schools, please let us know).
ACM Summer School on Recommender Systems
The ACM Conference on Recommender Systems occasionally has a summer school prior to the main conference and in addition to a doctoral consortium.
ACM Latin American School on Recommender Systems (LARS 2019)
Machine Learning, Information Retrieval, Statistics etc. Summer Schools
While the number of recommender-systems summer schools is limited, we would recommend considering some of the various summer schools on related topics like machine learning, data mining, or statistics. Many top universities and other organizations offer such summer schools. While we have no particular recommendations, Google may help you:
Machine Learning Summer Schools: https://www.google.com/search?q=machine+learning+summer+school
Statistics Summer Schools: https://www.google.com/search?q=statistics+summer+school
Data Mining Summer Schools: https://www.google.com/search?q=Data%20Mining%20Summer%20Schools
Information Retrieval Summer Schools https://www.google.com/search?q=Information%20Retrieval%20Summer%20Schools