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:

1. 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.
2. 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.

In my university, we have the students seminar around 12pm with pizza at the end. Using a time slot when most people are free usually helps. And although sweets are good you can't survive on that, free lunch on the other side is always a plus.

Else we also have seminars friday around 4pm with snacks and beers afterward. It is a time when most people are not as productive as the rest of the week, and the ability to socialize afterward with the rest of the department is always a plus.

Of course, the best way would be to engage the leaders. Maybe invite them to give a talk and try to make it worth their while so they can see that the goal of those seminars is not only giving the talk but the discussions that can flow out of it.

I suppose good team leaders encourage their whole team to show up, while a few others have told me point blank that they consider it “wasted time”

First I wouldn't put a quality judgment on the team leaders. Hopefully all the team leaders are "good". Further, hopefully all the team leaders have done a cost-benefit analysis of their staff attending and have simply come to different conclusions.

The problem does not seem to be the junior staff, but rather the example set by the senior staff. I would argue that you do not want to encourage the junior staff to "disobey" their team leaders by offering sweets. The permanent staff needs to come to a consensus as to whether or not these meetings are useful and who should attend. The possible outcomes are as follows:

1. The meetings are a waster of time and should be canceled
2. The meetings are critical for all groups and attendance should be mandatory
3. The meetings are useful for some groups but not others ant those that want to attend should attend
4. The meetings are critical to some groups and require participation from all groups and attendance should be mandatory.

Once a decision is reached, it is the lab director (the person responsible for the 25 permanent staff) to see that it is carried out.