I really need some help answering this question. Could a man who lived in another town or part of Germany, for example East Frisia, about 1500 AD, go to Bremen for example and become a burgher there? What were the requirements he had to meet in order to become a citizen? Would joining a guild automatically give him citizenship status? What requirements did guilds have for those coming into cities from elsewhere?
As Bremen is only an example (random, as far as I know) given above, I will answer based on Reval, which was given city rights based on those valid in Lübeck (therefore Lübeck had its place as a court of appeals for any claims against Reval).
The situation that applied was that any serf who escaped and lived in the town for a year and one day would be free from any reprisal by the landlord -- which presumably answers the question as after this point these people would have been primarily responsible to city law and not a specific landlord. This was beneficial to the cities as they'd gain cheap labour.
The (German) saying 'Stadtluft macht frei' is used to symbolise this principle. The Wikipedia article, however, suggests that HRE law superseded that whilst I know of instances in 16th century Reval where the principle was still used to avoid serfdom. Therefore, I would think that either Lübeck's status as a Hanseatic city or the Freie Städte concept within the HRE allowed it and cities which derived law from it to maintain this concept.