From sona pona, the English–Toki Pona wiki

The English word "with" has multiple meanings that are phrased differently in Toki Pona.

For the sense of "using", use the preposition kepeken.

mi pali e tomo kepeken luka-mi taso

mi pali e tomo kepeken luka mi taso.

I built a house with my bare hands.

For the sense of "alongside", you can use multiple subjects connected by en. You can also use the prepositional phrase lon poka, "at the side (of…)".

mi en ona li pali e tomo

mi en ona li pali e tomo.

Me and them built a house.

mi pali e tomo lon poka-ona

mi pali e tomo lon poka ona.

I built a house at their side. (Could also mean "I built a house that is in their vicinity.")

For the sense of "including", there are other ways of phrasing it.

mi pali e tomo ni> zz ona li jo e supa

mi pali e tomo ni: ona li jo e supa.

I built a house that has furniture.