I find your Groups feature to be one of the most powerful features on WA when it is allowed to be used as a criteria. Please add it to the Availability options in all events. This would Events to be sold only to a select group of members. It has many applications: early registration for VIP's - open registration could be allowed by just a click of a few buttons, I see many clients creating numerous membership levels just to control different VIP levels; They could use just several but **** would get into different VIP groups with special registration rights. This can be managed by limiting viability of an event to a Group but now all other members can't see the special events they would be allowed to attend if they would make the jump to be members of that VIP group. Basically, I think your groups feature allows us to customize our websites and therefore groups should be used everywhere access control is restricted.

Have you solved this yet?

To me, a simplest of solutions would be

Event Registrations allow a drop down option that says, "People who register and pay for this event are added to Group XYZ" and the drop down allows you to choose which group that gets added if they are not already in it.

website Groups added to registrations

Comment from client:
"I am the primary admin for two organizations that use Wild Apricot (WA). The two organizations are the Utah Rock Art Research Association (URARA - account 202570) and the American Rock Art Research Association (ARARA – account 219734).
Both groups hold an annual conference with roughly two hundred attendees. Each conference also features a series of field trips. Because our field trips are guided hikes they have small registration limits, so we tend to have a lot of field trips. So at each conference we tend to have 20-30 related field trips.
Our field trips are incredibly popular with our members. It is important for us to make sure that only people who have registered with the conference are participating in the field trips. Ideally we want field trip and conference event registration to be linked. In computer speak we would like a “If registered for conference, then can register for field trip, else not.” To accomplish this we use the WA registration code system for the field trip event sign-ups. So when someone signs up for the conference we have customized their registration email with the registration code they need to sign up for the field trips. The problem with this is that the code has to be the same for everyone who signs up for the conference. So a conference attendee can simply email the code to a friend who doesn’t want to attend the conference but wants to go on a field trip.
I’m sure you have a bunch of ways of resolving this problem. I’ve only thought of two. The first would be that registration codes are custom generated for the participant. That sounds complicated to manage on the field trip event end of things. The simpler method would be to make events accessible to membership groups and not just to membership levels. Let’s say that when someone registers for the 2017 conference they are automatically enrolled in the “Conference 2017” group. Then have field trip registration limited to that group. Group membership seems more flexible to me than membership level which is an event registration option now. We already have levels such as “single”, “family”, and “student.” Since a member can only be a part of one level, it is impossible to assign someone to a “Conference 2017” level. But groups are a much more flexible option."