If dealing with adults you do not know well, mostly the formal version ("mit Ihnen") will be necessary.
(wiki, to be short: "Usage varies in the German-speaking world when addressing a group containing both du and Sie persons from the speaker's point of view. Some speakers use the informal plural ihr, others prefer the formal Sie and many, concerned that both pronouns might cause offence, prefer to use circumlocutions which avoid either pronoun.")
It is also possible to use the informal address - Euch - with adults, but this is not recommended for non-native speakers before reaching a certain level of proficiency; sometimes it is difficult to tell what is appropriate.
For example the boss could address his workers with "euch" - and this could come over as patronising (= negative); but it could also be interpreted as the boss being on (almost) the same level as his workers. Only external context could decide which would apply in a certain situation.
So non-natives best should avoid "Euch" with adults except if they are very sure of the impression they will make in a given context.