That's why the discussions are had in an OOC thread - makes it much easier for everyone to see if something is happening or if new people could easily join. Often if you can see what other people are doing, it's easy to tell if your character would fit in or not. If you think you'll fit, you post there and the first person to see it makes the decisions or whoever was leading the current events says "Yeah sure" or "Give us until we get to the water - got something planned!' or whatever. It depends on individual circumstances.

For example, you are leading shion's character across the city presently, so if somebody wanted to join the two of you, you would probably be the one to answer since you are more aware of where you are leading her etc. You might not have a solid plan, but you are sort of in charge of where her character is going in current circumstances. If the two of you were traveling more equally, either or both of you would. Case by case basis sort of deal.

In the past we've always just had a thread where we update as we have ideas and say "Hey, X and I were thinking about going to Y or doing Z next" and somebody else might think "Hey that sounds awesome - can my character join you on that trip?" and they'd be like "Yeah sure - just let us finish this conversation about whatever first. It's important for my character's backstory." It was always super informal.