As someone who has been putting a lot of effort into planning private tours for my upcoming cruise and looking for others to share the tours with I have to say first come first serve...
Unless it is stated from the beginning that there are rules as to who can join etc.
I chose to set everything up in a spreadsheet that everyone on our cruise can see. It shows availability and who is signed up for what. We also asked everyone to provide an email address. When a member wanted a tour posted they posted the info for me or emailed it to me. It is up to the members to keep up with the threads, you miss a day and sometimes you miss pages of posts...some days you miss nothing. You snooze you lose... My cell alerts me whenever there is a new post... That is how active I have CHOSEN to be on my rollcall... You can CHOOSE to be active or not. I also do not think it should be frowned upon for being that active!! Again it's a choice. My phone alerts me, I choose sometimes to respond right away, other times I choose to ignore it. I do not think it is fair that just because someone booked their cruise later than someone that they should not have access to any openings. Now you cannot complain if you are late on roll call and the private tours have all booked up... that is totally different.
I arranged my own tours for myself, and if others wanted to join that is great! However, as others have said, there will always be those bad seeds in the group. As someone said they seem like they will take over... We have group leaders and it is their job to be in charge, if you want to be in charge of a group, take the initiative like many others do and set up your own tour! ;)
I prefer to do private tours as they are typically cheaper and way smaller groups. I made sure I looked up reputable companies, that if someone no-showed or canceled at the last minute that it would not affect me! The company's take a visa # etc for booking purposes only, you no-show, they charge you...then I don't get stung in the end.