This is known in the industry as "top-up" and is a common practice on all scheduled airlines.
As soon as it is clear that the group is growing and you need additional places, contact Intertours immediately.
The additional seats required will then be requested from the airline at a binding group price. In advance, you can already be given a tendency as to whether the group price for the additional seats is possible or not, or whether the utilization of the machine allows additional seats at all.
After your confirmation, Intertours will issue an extra invoice for the additional person(s) according to the down payment option.
Now there are 3 possible outcomes - let's start with the simplest one:
a) The request is positive and the additional place is confirmed at the group price, the booking is immediately binding.
b) The request is positive, but the additional seat can no longer be confirmed at the group price, only at a higher price. You will then be informed of this and you can decide whether the place should be booked at the higher price or not. The booking is only binding after your confirmation.
c) The request is negative and no more space can be offered on the group flight. Perhaps, however, there is still space on one route of the group flight, in which case a request for changed conditions can be started again, see also 3.b). Or Intertours books the person completely separately and detached from the group at the current price, see also 3.c).