FWIW - Gath.io and Friendica are both currently using an RSVP model for events that isn't actually defined in ActivityPub (or at least this was the case a week ago - things can change rapidly in this space). Technically sending an 'Accept' should only be in response to an 'Invite/Event', not a 'Create/Event'. This is spelled out in AS vocabulary definition, but many of us (including me) got it wrong the first time through and everybody copied each other to get it working. So now we've got a bunch of projects that are compatible with each other but aren't actually compatible with the standard. To remedy the situation my own software will accept an RSVP response to a Create/Event but will never generate this activity or response and will only RSVP to an Invite activity. You can choose to do it either way but I anticipate that eventually we'll all have to follow the spec or we'll end up with two incompatible event systems.