We had a similar situation when we opened- we didn't really have a visitor rule, and found that people would bring a visitor in and then the next time we'd see the 'visitor' was 5pm when they walked out. We've been through a few policies since, but like Jeannine we realised that our front room was the key. We have a front room which seats around 10 people, plus a breakfast bar which serves as reception. At a Town Hall we floated a proposed policy where as long as members were checked in, they could have visitors in the front room- on the proviso that visitors were always checked in, and that the member was responsible for their visitor's behaviour, and got them any drinks etc rather than us doing it. We then had a policy that a visitor who walked through the members door into the main space got charged to the member they were with as a day pass.
So we've ended up with the following options for members and visitors;
1. Member checks themselves in to the hub using their swipe card. Members manually signs visitor in. Both people sit in front room. Member can get visitor a free drink. Extra cost to member = zero (this means we lose less people to the 5 coffee shops opposite us). Means that the member has to take their chances with respect to whether there will actually be any seating available at the time their visitor arrives. Also not much use for private meetings.
2. As above, but member takes visitor through to the members area. Member (discretely) gets charged a £10 visitor day pass. More room in the member areas, plus member meeting rooms to use for private meetings, though these aren't pre bookable and again the member takes their chances. Visitor can stay afterwards and work if they wish, but the member is responsible for them.
3. Member books a £10/hour meeting room, guaranteeing privacy and seating for them and their visitor.
We always allow 15 mins for a member to show any visitor around without charge, which allows for people wanting to show their significant other/kids where they work
This policy works incredibly well, and we've not had a single complaint or moan since we agreed it (well apart from the fact that two members left immediately, and they were the ones who were causing the issues, so we assumed that would happen)