If you need to convince your manager to send you and your team to TestBash, you need to get a new job with a better manager. Your manager should be actively looking for personal development opportunities for everyone they manage. TestBash is one of the best known conferences and any competent manager of testers should know about it and encourage attendance.
It’s obviously a bit late, but job applicants should be asking these question at the job interview. Will the company pay for attendance at all the important conferences and events? Will they give you time off or do they expect you take it as holiday (yes, I know companies that actually do that)? What other personal development activities will they pay for, such as books, online courses, classroom courses etc?
Sadly, there are a lot of bad managers out there, focusing only on getting stuff out the door and not caring about the development of their staff. But there are some good ones, so go and find them.
I wouldn’t call these tips, but more things to think about when asking to go to a conference (such a Testbash):
1. Is there dedicated training budget per employee? And, is the total cost of attending the conference within budget?
If you’re at a company where they have dedicated training budget per employee and the total cost of Testbash is within budget, then convincing your manager will be easier IMO.
If not, then you can still ask but note: their ability to say yes may depend on how much red tape/bureacracy they will face,
2. Be specific about which talks/workshops you want to attend and how they will benefit the company
This point is expanding on the 2. top tip already suggested. If you can, do your research into the announced talks and workshops so far, then tie that to what your company is working on currently or even better, OKRs/team priorities and initiatives.
It’s one thing to say your company will benefit, but it’s a better sell if you can tie it to something concrete.
3. Ask your manager you would really like to attend (for reasons stated in #2 and ask them what your options are to make that happen so that the company would support you and don’t have to take it as a vacation day.
Instead of focussing on trying to make it work, express your desire to your manager and then say “how can we make this happen?”