Client-to-guide ratios are set per experience. On an existing booking there is no way to change the ratio — not from the booking, and not from the trip. So adding a client to a trip that is already at its ratio means the booking requires another guide. On the trip itself there is no empty client slot to click, because slots come from the booking.
The Workaround: A Duplicate Experience at a Higher Ratio
Duplicate the experience and set the ratio you need. If your standard trip is 2 clients per guide, keep a 3 clients per guide version for the cases that call for it.
Duplicate the experience and set Number of clients per guide — for example, 3 clients per guide. See Duplicating an Experience.
Open the booking, click Edit, and change the experience to the duplicate. See How to Edit a Booking in Brindlechute.
Add the client. See How to Add Clients to a Trip as an Outfitter.
💡 Clients will not see this experience online. An experience that is not in a group does not appear in your online booking modal. Leave the duplicate out of any group and it stays internal — you use it case by case, and clients cannot find or book it themselves.
Keeping the Price the Same
Changing the experience recalculates the total. If the price should not change, override it on the booking. See How to Override Prices on a Booking.