There are several synonyms that can be used for the phrase "come in". Alternatives include "enter", "step inside", "go in", "cross the threshold", and "come through the door". Other options might depend on the context for which the phrase is used. For instance, if someone is speaking over an intercom, "buzz in" or "ring the doorbell" could suffice. Similarly, if someone is being welcomed into a room with a gesture, "join us" or "join the group" may be appropriate. Overall, the key is to choose a phrase that accurately conveys the intended meaning in the context of the situation.