Learn Arabic (Modern Standard): الضيف في غرفتكم.

The ending ـكم (-kum) is the attached possessive pronoun for "your" when speaking to a group of people (plural masculine or a mixed group). You attach it directly to the end of the noun to show possession.

The word for "room" is usually غرفة. What happened to the ة at the end?

When you add an attached pronoun like ـكم to a noun ending in a taa marbuta (ة), the letter "opens up" into a regular taa (ت). That is why غرفة changes its spelling to غرفتـ before adding the suffix, making it غرفتكم.

Where is the word for "is" in this sentence?

There isn't one! In Arabic, simple sentences describing where something "is" in the present tense don't use a verb. You simply put the subject (الضيف, the guest) and the location (في غرفتكم, in your room) next to each other.

How is the first word, الضيف, pronounced?

It is pronounced "ad-dayf". The letter ض (daad) is a "sun letter." This means the "L" sound in the word for "the" (الـ) is skipped, and you double the ض sound instead.