Yes, except the verb -ambia almost always has an object pronoun indicating the party who is told.
Waambie = Tell them.
Uwaambie = You should tell them.
Waambie (?) = They should tell (?)
Wakuambie = They should tell you.
No. "They should tell" would be "Waambe". The word kuambia/kwambia is the applicative form of the verb kuamba/kwamba. It's a somewhat advanced topic, but basically, the applicative form changes the verb, so that the indirect object becomes the direct object. Moreover, the new applicative form would demand that the indirect object should be mentioned.
For example "Nitakuambia kisa" would mean "I will tell you the report/story", while "Nitakiamba kisa" means "I will tell the story/report" without mentioning to whom I'm telling it.
Another example of this is the difference between 'leta' and 'letea'. The last one is the applicative, and means 'to bring for (someone)' while the basic form, 'leta' means 'to bring (something)'.