When working with agile (or any methodology) I would suggest making your testers part of the team, rather having a separate QA team which is isolated from all the work. (This also means sitting the testers with the developers if you are in the same office.)
In your points above you have listed a lot of tasks, these tasks are not the sole responsibility of the tester, the whole team needs to be getting involved.
prepare stories for the future sprint with business analysts
Preparing stories should be done by the product owner and if this is not possible, then the whole team should be sharing the responsibly and ideally doing this before any planning session.
Discuss stories of the current sprint with developers and BAs
This shouldn't be needed in agile, everyone should have been in that planning meeting, so have a good idea of what is going on to start with.
keep an eye on broken build on CI
Just as much the dev's responsibility as the testers.
Work with product owner for the demo
Again, let the developers get involved with this, they are the ones who worked on the stories, so they should also teke responsibility for demoing them. On a previous project I was involved in, each developer was responsible for putting a short demo together of each of the stories that they worked on and would take it in turn to show these to the product owner. The tester would still be in the room and give an update on the QA side, but it was the developers who actually did the demo.
You get the idea, share the workload ....
The following article may also interest you that we have written https://blog.testlodge.com/the-role-of-qa-in-agile/ which goes into more detail on how a tester can get involved with a QA team when working with agile.
If you do have a team of many testers, then one thing you could do to keep the communication going between them is let them have a scrum of scrums, where they all meet briefly and provide each other with an update. (This doesn't have to be once a day, experiment with different time periods and find out what works best.)