Guest is the right corresponding term and this is what I would personally use. It is commonly used in a variety of situations:
a person who is staying with you, or a person you have invited to a social occasion, such as a party or a meal (Cambridge)
Guest is very used on sites to differentiate between members who have an account and (anonymous) guests or guest users who do not have an account, or have a different type of account. So guest would go well in the chat situation as well.
But there is also the slightly more technical term
a person who is invited to something:
This one is more formal, but it might express better what you need, as it focuses more on the fact of being invited.
In law, invitees and licensees are both guests, but:
The main difference between an invitee and licensee is that an invitee has been invited for business purposes while a licensee is someone there for social purposes or a reason unrelated to business. (burkelaw)