Use of the word 'together' as in the Norfolk dialect
Solution 1:
If it’s any help, the OED’s definition 2e of together confirms this use. Its earliest citation is from the first part of the nineteenth century. This subsequent citation from J G Nall’s book ‘Great Yarmouth and Lowestoft’ of 1866 suggests that it may have had a singular form:
It has been wittily observed, that . . . ‘together’ is [the] plural [of ‘bor’][a single person, male or female, being addressed as bor or ‘bo’, two or more persons as ‘together’].
Solution 2:
The Scandinavian word for ‘together’ (Danish and Norwegian sammen, Swedish samman or tillsammans, Icelandic and Faeroese saman) can function in that way to a limited degree.
If I’m understanding your examples correctly, the sense is really ‘together with us’ or ‘along with us’, which does work in Scandinavian languages—sometimes.
Would you like a cup of tea, together ≈ (Da/No) Vil du have/ha en kop/kopp te sammen? / (Sw) Vill du ha en kopp te tillsammans?
Come in here, together ≠ (Da/No) †Kom herind/inn her sammen. / (Sw) †Kom in här tillsammans.
The first works okay (but would sound more natural, as in non-Norfolk English, if phrased like “Should we get a cup of tea together?” vel sim), but the second is quite impossible in the sense intended: it would unambiguously be understood as asking multiple people to enter the room in unison.
A perhaps somewhat related concept is the Insular Scandinavian (i.e., Icelandic and Faeroese, but not Danish, Norwegian, or Swedish) ability to use a plural pronoun + a name (or similar noun phrase) to express that the plural pronoun refers to the person indicated by the pronoun’s number plus the name/noun phrase mentioned:
(Ic) Við Jón hreinsuðum til í húsinu. = We John tidied up the house. = We [=John and I] tidied up the house.
This is not possible in any other language that I know, but seems to be a related phenomenon.
If that is a misreading of how the word is used in Norfolk English, and you mean that ‘together’ is simply used as a kind of vocative to refer to more than one person (similar to how American English would use ‘you guys’), then the parallel to Scandinavian is less straightforward.
All the Scandinavian languages can optionally add ‘together’ to the word for ‘all’, either singular or plural (neuter singular: Da/No alt, Sw allt, Ic allt / plural [masculine]: Da/No alle, Sw alla, Ic/Fae allir) to make a lexeme that simply means ‘everything’ (singular) or ‘everybody’ (plural)[*]. But this isn’t particularly vocative, and it isn’t really too different from ‘all together’ in English, only more lexemic.
I’ll leave the German links to someone more versed in that tongue than I am.
[*] The more common word in Swedish is alltihop (singular) / allihop (plural), which is basically identical: ihop (etymologically from i hop ‘in (a) group’) is just a different word for ‘together’ that’s only used in Swedish and sometimes in Icelandic (and Faeroese?). Allir í hópi is also quite common in Icelandic, though not (as in Swedish) more common than allir saman.