Plus one / John Hare.

Enhanced descriptions from Syndetics:

When Agnes decides to host a tea party, she finds it hard to shake a most unlikely guest . . . and his even more unlikely friends.

Agnes is new to town and wants to make friends, so she invites some girls in her class to a fancy tea party. When there is a knock at the door, she expects to see them-not Dave from next door. This isn't what she had in mind at all. . .

"The invitation says plus one.
This means you need to bring a friend.
And since you don't have a friend with you-goodbye."

Agnes is sure that will be the end of that, but Dave is persistent, and before long he's bringing one friend after another as his plus one. Pierre of the North, Esquire, a goose who Agnes deems too loud; Fred, a cactus that's too prickly; Dr. Scalywiggles, a legless lizard she mistakes for a snake; even his mom, who makes great French toast and lets him win at tag. Alas, no grownups are allowed at this tea party.

But as the minutes pass and the other guests don't show up, Agnes is left with nothing but her tea set, watching Dave and his friends play tag. She works up the nerve to ask if she can join in and receives a resounding yes- and when the original guests arrive (late because of soccer practice) everyone heads inside for tea, no species excluded.

In this fun and funny tale, John Hare, author and illustrator of Field Trip to the Moon , Field Trip to the Ocean Deep , and Field Trip to Volcano Island , uses his signature wit and buoyant illustrations to share the timeless message that sometimes new friends turn out to be someone unexpected.