Assuming you're running your shop as a business, there's a simple analysis you can do:
Question #1: What business issue/opportunity are you trying to overcome/exploit by adding an agency?
Question #2: What are the time, cost, and logistics requirements to add another agency?
Question #3: Is the potential upside worth the time, cost, and logistics?
If you don't have an answer to Question #1 right on the tip of your tongue... don't even bother with #2 and #3. The fact that this question is borne of YOU having been approached by OTHER AGENCIES in THEIR efforts to get YOU to affiliate with them tells me that these other agencies are trying to exploit a business opportunity for THEM by getting you to add them. (Unless you're in a situation where you have a significant portion of potential customers telling you that they are going elsewhere for training because they don't want to take PADI courses... and you are actively seeking other agencies.)
Assuming YOU have a compelling business reason to go down this road, you need to consider the following at a minimum:
- Crossover training costs (for each of your 7 staff folks, or however many)
- Crossover training time/logistics (for each of your 7 staff folks, or however many)
- Crossover material costs (for each of your 7 staff folks)
- Professional member fees (for each of your 7 staff folks, or however many
- Inventory carrying costs to have materials for the same course(s) for multiple agencies in stock
- Logistical impact of running courses (concurrent?) for the same courses across different agencies
- General Business inefficiencies associated with dealing with more than Plusagency (Suppose you have potential OW students - right now you put them in a single PADI course. If you also offer NAUI... do you possibly end up with 2 students wanting a NAUI course and 2 students wanting a PADI course?)
Lastly, once you've figured out those business impacts... you need to determine what the potential upside is, and whether it's worth the cost/time/hassle. If you go through everything above and end up pocketing only a few dollars of incremental profit per year... it's not worth it.
Also, consult your PADI agreement. I see you are currently a PADI 5-Star Dive Center. I believe that one of the requirements for being a 5-Star Dive Center is that you are not permitted to offer any course from another agency for which PADI has a course. For example, you can offer SDI Solo Diver and remain a PADI 5-Start Center, however if you also offer SDI Open Water you are no longer eligible to be a 5-Star Center. Naui Cave - no problem. NAUI Cavern - nope. Etc. I'm pretty sure it's the 5-Start status that is at risk here. Note the difference in the descriptive language on the PADI site, with my emphasis added:
PADI Five Star Dive Center:
"...PADI Five Star Dive Centers embrace the PADI System of diver education..."
PADI Dive Center
"...These dive shops use the PADI System of diver education...