You asked "do I advertise Beginners Welcome"
It seems to me that there are 3 levels here.

1. The sort of session that is for a particular kind of tune and expertise, and if you don't play that then you are made aware (however politely) that this is not really for you.

2. The session that takes whoever comes, gives them a hearing, and supports them if they need.

3. A session that specifically says 'Beginners Welcome' which could mean that you are prepared to play standard tunes slowly for a good part of the evening, and maybe had someone who was prepared to support or lead tunes the beginners had ALMOST mastered.

One way to go might be to say 'all welcome', then when you see who turns up a few times, ask if a 'beginners half hour' to kick the evening off would be a good idea, if you had someone prepared to lead it.
I used to sometimes go to 'Nellies' folk club when I was in Beverley, and Ollie started a 'learn to play some tunes from the David Oliver CD' half hour. It was really useful to get into playing.