This phrase is casual, more common in spoken or informal writing among friends or peers. Are you free to knock around together this weekend?
Pro Tip 2/3
Close Friendship
Implies a friendly, often close relationship with the person you are spending time with. My cousin and I used to knock around together during holidays.
Pro Tip 3/3
No Specific Activity
Used when the activities done together are not planned or organized; they're spontaneous and casual. We'd just knock around together, talking about everything.