You can use the /who command to check out the online membership of any kinship you're invited to or interested in. It's a good idea to do this at your main play time to see what the membership is when you play, especially if you don't mostly play server primetime. You'll need to expand the levels displayed from 1-75 to see everyone online. Of course a kinship with a lot of players online is not necessarily that active in terms of playing together, grouping up, or even talking in kin chat, but it at least gives you some indication.
A kinship with people near your level (especially if it doesn't have a high turnover rate) can be great as it's easier to find groups at your level. If you're looking to do end-game content, you'll need to be in a kinship that plays the end game together. Some do more PvMP. Some are altoholics. Etc. Good luck!
Pierced by the friendship of the mortal races.