The transfer of money is not a requirement, so you can be a stammis på biblioteket (a regular at the library) or a stammis på forumet (a regular on the online forum). It's a noun, so you could say something like jag är stammis på baren (I'm a regular at the bar) or här är han stammis (he's a regular here). Stammis comes from the compound nouns stamkund (regular customer) or stamgäst (regular guest) but those two words sound much more formal than the colloquial stammis with the typical -is ending that pops up in a lot of Swedish slang words.