How to encourage seminar attendance?
So, I am wondering what we can make to help increase student turnout. What do you use to attract people to seminars?


Short answer: Make the seminars useful for the group members. 

First, the diagnosis: The group members are probably too narrow-minded and do not understand that getting insights from currently irrelevant topics does in fact become often very useful in the long-run. The group members seem to optimize in a greedy manner for their short-term interests, shooting themselves in foot in the long-run. It seems, they do not understand that seeing connections between dots at some future timepoint is much much easier if you saw the dots and their contexts before. But this does not come by direct explanation, they need to realise it by themselves. It's your task to set the example and at least showing how at least you benefit from the seminars. This is a long-haul task and has to do with your general attitude to world. In the short term, you can perhaps do the following:


push for all group members (including the professor(s)) giving conference rehearsal talks - if you are in an area where going to conferences makes a difference. At the talks encourage giving the speaker not only content-relevant feedback, but more importantly methodological feedback on how to speak.
invite external speakers and actively support networking of the group members with the speaker. Especially in informal interactions (which are often started by interactions during, or right after the talk), people tend to find common interests and receive feedback on their own work. Possibly start even a small collaboration. The idea here is to, over time, show the group members that attending tangentially relevant talks is useful for cross-breeding of ideas.