I've just been reviewing the posts I've contributed to during the past month and spotted that you've posted a further query. I hope that this reply isn't too late:
Unless you're prepared to get your group to walk to the pick-up points for private hire companies (in car parks 1 & 3), you'll have to use one of Glasgow Airport's authorised taxi firms. (They're the only cabs which are allowed to pick up from the rank at the front of the terminal). You can get a quote (and information about booking, if required) by contacting Cabfly (which is the taxi management company for Glasgow Airport, rather than a taxi operator) on 0141 848 4588.
Alternatively, contact the main company which is authorised to use the pick-up point at the front of the terminal. For their details, including a map which shows how close the taxi pick-up point is to domestic arrivals, click here:
http://www.glasgowtaxis.co.uk/services_airport .php
Chris