you can do a search for 'quilt retreats', check with your local quilt shops, browse the on-line shops (joining the 'fabshophop' puts you in touch with about 130 quilt shops across the country-many of which sponsor quilt retreats throughout the year)
as for- inviting yourself...most of the time everyone (even non-quilters) are welcome ---but space is limited- you have to sign up & pay for your 'spot' before the retreat. start with your local resources (quilt shops) also, favorite authors/designers - they often have retreats listed on their calendars/tours too. retreat's vary in many ways- length (weekend up to a week or two) price, what all it includes, what you need to take---there are retreats in woodsy, vacation places, on board cruise ships, in big cities- some sponsored by a fabric designer, pattern designer, big company or little quilt shop- sometimes they are just (sit & sew) retreats where they do not provide anything, or teach you anything- you take your supplies, your project and get away from home for a few days- to sit & sew at your leisure. (sometimes called a 'quilt camp) sometimes sewing machine companies sponsor retreats- allowing you the opportunity to use their new machines and see what all the bells & whistles will do...there are lots of options.