It's better to restrict by theme rather than by medium or length of practice, for example, as themes allow a greater flexibility in who can attend. You may not have speakers from outside the group, but this can frequently engage members more than hearing from within the group all the time.
Your speakers need to be appropriate for the members you have chosen. Are they at a similar career level to the members, and does their work fit with your theme? You do not need high-profile speakers, but they must be interesting and articulate so your members keep coming along. You might find speakers through your own personal professional or social networks, or through your members, or even via advertisement on a studio notice board or email mailing list. Find out what equipment each speaker needs well in advance of their talk.