1. let enter (Sounds VERY formal and only used in contexts like opera and so on. Has a notion of welcoming, rather than giving permission. The noun is more common.)
2. to get involved with, to engage, to say yes to ("sich+Acc einlassen auf+Acc" - in the context of deals or offers of some kind, where you are a bit skeptical. Also used for engaging with people in the sense of a relationship - business or private.)
3. to fill, to let in (ONLY for bathwater or water in the sink. The water/bath is the direct object, usually, but it can also be the bathtub.)
Opposite (closest): ablassen
4. to set in, to embed (In the sense of arts and crafts. Think of a gold smith slowly putting a stone into a socket. ONLY for such contexts, not to be used generally.)