I am assuming you mean your radios are not narrowband capable, and you do not want to replace them?
If you have a license (Itinerant or otherwise) you can modify it for narrowband and have your radios reprogrammed if capable. But again, I guess I need to assume you are asking because they are not.
The only "business" you can use GMRS for, is a family business, where all operators are family members and at least one has a GMRS license. (If you had an Itinerant license, this is where you may as well stick with what you had...versus changing only to pay for a new license and radios). So, again, if I assume your club is not family, and this is a business venture. You could not use GMRS. If it was not business (IE make money - for profit - paid to do it) you could possibly use it, but ALL members would need to hold licenses.
Getting too technical? You may want to share some more info. What kind of event, is the club a non-profit, what kind of radios do you have now, what frequency, what type of license do you have (Itinerant or coordinated....whats your call sign), etc.
I think myself and others can offer some suggestions.
In GMRS you do not have to use repeaters, there are low power channels, and high power channels, plus repeater pairs. If you were using only handhelds before and that worked, then you don't need a repeater anyway.
There is MURS (VHF - 2 watts) for business or personal, no individual license required. Also, FRS, which share some GMRS low power channels, but is licensed by rule as well (no indiv license) but on FRS you have people trying to use it for business, kids, legit people, etc. But, there are 14 channels and lots of privacy codes.
So, tell us more and we'll try to help.