Alex, it still problematic, don't know if it depends on upgrading, but for sure after it, so....

check, please invitation to secret group, there is no option to enter it

After updating three .php files from GitHub feed behaviore looks fine but :

1. User A opens Secret Group

2. User A sends invitation to user B

3. User B get the email and click enclosed link to group

4. User B can not enter becouse of "Access denied. This is private content."

Thanx in advance