I know which forums specific user read - how to cluster the data?
There are different approaches from easy to complicated
Similarity-Based

Distance or similarity matrix is calculated for different topics (forums) and if two topics are more similar than a threshold, you suggest them to the users of the other one. For this, you need to gather all texts of different forums and vectorise them (from TF-IDF to Neural Embedding). Then define a similarity score which tells you how related topics are. Then suggest them to users.
Second way of doing the same is to calculate similarity of different topics based on the number of mutual readers. Then doing the same as above.

Community Detection (Clustering) in Bipartite Graphs
Familiar yourself with the concepts of Community Detection and Bipartite Graphs. Then apply this clustering on users based on the forums they read. Select top topics inside each cluster of users and suggest them to all members of that cluster.
Link Prediction in Bipartite Graphs
Familiarize yourself with concept of Link Prediction. Applying link prediction on your problem gives you most probable links between users and forums.