When a guest has unnamed plus one guests, you can admit them individually from the guest profile.
Steps
Select the Guest list tab and search for the guest.
Check the plus one count on the right side of the search result. For example, (0/2) means two plus ones, none yet checked in.
Tap the guest name to open their profile.
Tap the Plus 1 (+) button to admit each additional guest one by one.
Tap the back arrow at the top left to return to the guest list.

