horn in on (something)
To join something when one is unwelcome; to intrude or encroach. Ugh, my little brother keeps horning in on all the things I try to do with my friends.
horn in on
Intrude, join without being invited. For example, She has a rude way of horning in on our conversations. This expression alludes to an ox or bull pushing in with its horns. [c. 1900]