We've been having this issue for now a month, me and my brother. Where we would send each other invites, but nothing would appear that'll enable us to accept. We would circumvent by joining directly through the PlayStation party, without needing that said invite.
With the new update, that option isn't available anymore. In the General Settings, it shows us that it's invite only. We tried sending invites by Ubisoft Connect, only to give us a server connection error. [8-0x00003700]