If you're 'chairing' the meeting, then first things first - introduce yourself, with a very brief background of your modelling experience/interests, then invite the participants to do the same.
Having established who's who, what's what, and what the level of experience is (on average), then discuss what the attendees expect and require from the meetings to follow.
From that, draw up an agreed rough outline plan/agenda for future meetings, to include a programme of sorts covering the requirements and expectations gleaned from the first questions.
Before any basic construction begins, for those new to the hobby or fairly raw beginners, decide on a simple, inexpensive kit which all can build (in stages) at successive meetings, with the aim being for all to finish their model at the end of the agreed period (of meetings). This item should be just one in a varied programme.
When commencing construction, emphasise the need for planning the build, which includes thorough study of the instructions and construction steps, checking for possible problem areas by test fitting, planning the painting stages and so on.
This all assumes that, in 'normal modelling', research into the subject has been undertaken first. (explain the need for this - for example, to avoid errors in colour schemes, markings, where an example might be a Bf109G6 in Battle of Britain colours, finished this way due to the modeller not having the knowledge of the history and service of the subject to portray an accurate scheme, where research would have provided that information, thereby avoiding a possible expensive mistake, and, more importantly, avoiding possible disappointment or discouragement. ).
This should be mentioned in the first build session, but covered by the fact that the subject/colour scheme etc have already been decided upon, and therefore the research is a given (i.e. provided by the 'instructor').
With newcomers, or even those who have a few models to their credit, tactfully explain that the aim is to get the basics right first (planning, parts preparation, joints and seams, detail painting, finish painting, decals etc), and that subjects such as 'weathering', extra detailing and the use of after-market products such as resin and pe can come later. And the catch-phrase (courtesy of Wojtek) - 'Modelling is not a race'.
The first part of this first meeting (the intros and discussions), will take up the bulk of the allotted time, but will create interest and a foundation to work on, which can of course be changed or adapted as time progresses. Better to plan it now, than founder about and have potential 'members' lose interest.
And, of course, the existence of this fine forum can be stressed !