Guest Users Enabled but Cannot invite to Group Chat

[AshSmith-Incodia]

I have enabled guest users in our Teams Admin Center and said guest user can be seen in a personal chat (I.e. I can invite/add them to a group private chat) but adding them to an actual team (So right click and add members) they do not seem to appear in the list when searching for them or pasting their email address in.

Has anyone encountered this issue before?

[#2]

@AshSmith-Incodia 

I am not sure if you turned on the right option, maybe you should check this again.

Did you enable org-wide settings > external access > the first option (on)? Are there any domains blocked or allowed in the area below?

Also for Guests accessing Teams you have to enable org-wide Settings > guest access > allow guest access in Teams and in the same site the chat option in messaging. 

[#3]

Could you confirm where that first option would be located? I'm fairly certain I've enabled guests across our O365 environment but if there's another button I need to tick?

External access is on for both Users can Communicate with Skype for Business and Teams users
Skype for Business users can communicate with Skype Users
With no domains blocked.

Guest Access
Allow guest access in Teams is enabled.

[#4]

You cannot chat with a guest user via group chat unless you invite them first to a common team that you share with that user.

External chat users cannot join group chats either. With native chat coming to Teams soon I believe that will change thou.