I would simply contact your manager in writing stating the importance and the possible benefits of the training for the company as well as mention that you are aware of the  fact that the yearly consolidated budget would be exceeded by X-amount of $ if all of your team-members will attend. 

In the next paragraph I would ask him if they would accept that the difference would be payed by the individuals attending the mentioned training and if they are keen that all members of the team would attend (Keep in mind that they might need some or all members in case of an emergency, important meeting, deadline, milestone that week).

Something along the lines of:

>Hey [enter name here]! Our team recently found out that there's a really useful training event named [enter name here] held at [enter location and date-range]. Our company and the team would really benefit from this training-event because [enter reasonns here].

>I am aware that the yearly budget for our team is exceeded by [enter $-delta here] since the sign-up fee is around [enterx-amount of $ here] for each person attending. Would it be possible that the difference is payed by each individual that would like to attend?

>Also, would the date [enter date-range here] interfere with any important, upcoming shedules, meetings or milestones/deadlines?

>Best, [enter your name here] 

**Note:** Make sure all of your team members are on-board with the idea and are also willing to attend and pay-up for the difference.