Why can't they join the group when invited?
When the greeter send a popup for them, they can't join because the system is laggy (Tested from Hungary and from Puerto Rico).
Do they receive an error message?
No error message, just we can't join, leave, create groups.
Can they join the group via Search?
The search is working, the website is working good. In the viewer the groups commands doesn't work good, because the system is very slow.
Can you provide the name of the group?
It's not relevant, all groups. Link: https://community.secondlife.com/t5/Controls/Created-a-group-but-it-is-not-showing-me-as-the-creator-or/qaq-p/2984699