Interestingly, I was working on this back on the vBulletin version of the Guild. Found this rough draft in my archive.
The simplest v1.0 solution might be to just let players submit a Join Request that GMs/Co-GMs accept.
For example, maybe you submit a Join Request when you submit your character sheet, and then GMs accept your request.
I've always wanted to have some sort of Roleplays <-> Participants link on the Guild which lets me build interesting things. One example that yall brought up is the ability to notify all the active participants of a roleplay.
The first challenge is to come up with a system that people are actually incentivized to use because it's useful.
If you have a roleplay with four players and only one person bothered to "join" it, then it doesn't really do anyone any good.
But it would also be a mistake to impose too much just-cuz rigidity on the freeform-by-design nature of the Guild.