The fact that they are enthusiastic enough, and sufficiently interested to take and post pictures should be sufficient to participate. This is not a forum for professionals but for hobbyists, amateurs, enthusiasts, a few who may make some sort of income from photography, and almost all of whom are interested in wanting to learn more.
You do sound like a good contemporary teacher. Everyone's opinions are equally valid as long as they're expressed in a positive tone.
Holding people to professional standards - and critiquing accordingly - may undermine the very purpose of the exercise which is that it should be a fun experience where people showcase what they do, get others to comment, and hopefully learn from the experience while enjoying it.
Telling people they did good and should just go and have more fun will undermine the entire exercise of a critique forum which is to help people improve on their photographic skills.
At this stage of the business, while I think it an excellent idea, if a culture of 'harsh but truthful' criticism becomes the norm on such a thread, those who simply enjoy photography as a hobby will politely withdraw and exclude themselves from the whole endeavour, leaving it to those who view themselves as professionally proficient and qualified to participate. Then, it will not be a learning process, but an esoteric sub-thread for a self selected photographic elite.
If a culture of "you did great, keep plucking away" becomes the norm, there is simply no point in even having the forum. People who want to grow as photographers will withdraw and exclude themselves leaving it to those who need their egos stroked with shallow praise and have no interest in actual growth as a photographer. It will not be a learning process but an esoteric sub-thread of people who believe everyone should get a trophy just for showing up.
People enter the weekly contest - which, remember, is not a professional setting, but one that is designed to be and intended to be fun, - in a spirit of cheerful optimism, and exchanges take place in a similar tone, one that prioritises constructive criticism. That allows a safe space where people feel confident enough to want to share what they have done.
And the weekly contest is fun. you nailed the intentions right on the head except that is no room for constructive criticism. The weekly contest has nothing to do with helping people grow as photographers. Nor should it, it serves its role well as it is.
At the end of the day, it is a venue for enthusiasts and hobbyists, not professionals, and a nicer and more welcoming atmosphere, to my mind, is a better place to hang out for hobbies, than one where strict standards of proficiency are applied.
And why do you think being a venue for enthusiasts and hobbyists should mean we don't want people to aspire to professional standards?